Sexual intercourse, also known as copulation or coitus, is commonly defined as the insertion of a male's penis into a female's vagina for the purposes of sexual pleasure, reproduction, or both.<ref name="Kar">Template:Cite book</ref><ref name="Collins">Template:Cite web</ref><ref name="britannica.com">Template:Cite web</ref><ref name="health.discovery.com">Template:Cite web</ref> The term may also describe other sexual penetrative acts, such as anal sex, oral sex or fingering, or use of a strap-on dildo, which can be practiced by straight and gay or lesbian pairings, or multiple partners.<ref name="Kar"/><ref name=health.discovery.com/><ref>The Gender of Sexuality: Exploring Sexual Possibilities; Page 76 – Virginia Rutter, Pepper Schwartz – 2011</ref>
Sexual intercourse can play a strong role in human bonding, often being used solely for pleasure and leading to stronger emotional bonds,<ref name="Diamond"> Template:Cite book</ref> and there are a variety of views concerning what constitutes sexual intercourse or other sexual activity.<ref name="Randall">Template:Cite journal</ref> For example, non-penetrative sex (such as non-penetrative cunnilingus) has been referred to as "outercourse",<ref name="Kamen">Template:Cite book</ref><ref name="Durham">Template:Cite book</ref><ref name="Sevenant">Template:Cite book</ref> but may also be among the sexual acts contributing to human bonding and considered intercourse. The term sex, often a shorthand for sexual intercourse, can be taken to mean any form of sexual activity (i.e. all forms of intercourse and outercourse).<ref name=health.discovery.com/><ref name="Randall"/><ref name="WHO, Sex">Template:Cite web</ref><ref name="www.ejhs.org">Template:Cite web</ref> Because individuals can be at risk of contracting sexually transmitted diseases during these activities,<ref name="Who">Template:Cite web</ref><ref name="CDC">Template:Cite web Also see Fact Sheet</ref> although the transmission risk is significantly reduced during non-penetrative sex,<ref name="Durham"/><ref name="aids.gov">Template:Cite web</ref> safe sex practices are advised.<ref name="Who"/>
In human societies, some jurisdictions have placed various restrictive laws against certain sexual activities, such as sex with minors, incest, extramarital sex, position-of-trust sex, prostitution, sodomy, public lewdness, rape, and bestiality. Religious beliefs can play a role in decisions about sex, or its purpose, as well; for example, beliefs about what sexual acts constitute virginity loss or the decision to make a virginity pledge.<ref name="Consent">Template:Cite news</ref><ref name="Intimate">Template:Cite book</ref><ref>Template:Cite journal</ref> Some sections of Christianity commonly view sex between a married couple for the purpose of reproduction as holy, while other sections may not.<ref name=Akin>Daniel L. Akin. God on Sex: The Creator's Ideas About Love, Intimacy, and Marriage. (2003). B&H Publishing Group. ISBN 0-8054-2596-9</ref> Modern Judaism and Islam view sexual intercourse between husband and wife as a spiritual and edifying action. Hinduism and Buddhism views on sexuality have differing interpretations.
Sexual intercourse between non-human animals is more often referred to as copulation; for most, mating and copulation occurs at the point of estrus (the most fertile period of time in the female's reproductive cycle),<ref name="Curtis">"Females of almost all species except man will mate only during their fertile period, which is known as estrus, or heat..." Template:Cite book</ref><ref name="McDonald">Template:Cite book</ref> which increases the chances of successful impregnation. However, bonobos,<ref name=FransDeWaal/> dolphins,<ref name="Bailey">Template:Cite journal</ref> and chimpanzees are known to engage in sexual intercourse even when the female is not in estrus, and to engage in sex acts with same-sex partners.<ref name="Bailey"/><ref name="Bagemihl">Bruce Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity (St. Martin's Press, 1999). ISBN 0-312-19239-8</ref> Like humans engaging in sex primarily for pleasure,<ref name="Diamond"/> this behavior in the above mentioned animals is also presumed to be for pleasure,<ref name="Balcombe">Balcombe, J. Pleasurable Kingdom: Animals and the Nature of Feeling Good. Macmillan, New York, 2007. 360 pp. ISBN 1-4039-8602-9 </ref> and a contributing factor to strengthening their social bonds.<ref name="Diamond"/>
Etymology, definitions, and stimulation factors
Sexual intercourse is also known as copulation, coitus or coition; coitus is derived from the Latin word coitio or coire, meaning "a coming together or joining together" or "to go together" and is usually defined as penile-vaginal penetration.<ref name="Kar"/><ref name="Kahn, Fawcett">Template:Cite book</ref><ref name="Coitus1">Template:Cite web</ref><ref name="Coitus2">Template:Cite web</ref> Penetration by the hardened, erect penis is additionally known as intromission, or by the Latin name immissio penis (Latin for "insertion of the penis"). Copulation, although usually used to describe the mating process of non-human animals, is defined as "the transfer of the sperm from male to female" or "the act of sexual procreation between a man and a woman".<ref name="Copulation1">Template:Cite web</ref><ref name="Copulation2">Template:Cite web</ref> As such, common vernacular and research often limit sexual intercourse to penile-vaginal penetration, with virginity loss being predicated on the activity,<ref name="Randall"/><ref name="Consent"/><ref name="Intimate"/><ref name="Carpenter">Template:Cite book</ref> while the term sex and phrase "having sex" commonly mean any sexual activity – penetrative and non-penetrative (intercourse and outercourse).<ref name="Randall"/><ref name="WHO, Sex"/><ref name="Cox">Template:Cite news</ref> The World Health Organization states that non-English languages and cultures use different terms for sexual activity, with slightly different meanings.<ref name="WHO, Sex"/>
Anal and oral sex may be regarded as sexual intercourse,<ref name="Kar"/><ref name="health.discovery.com"/><ref name="Randall"/> but they, as well as non-penetrative sex acts, may also be regarded as maintaining "technical virginity" or as outercourse, regardless of any penetrative aspects.<ref name="Intimate"/><ref name="Crooks">Template:Cite book</ref> Heterosexual couples often engage in these practices not only for sexual pleasure, but as a way of avoiding pregnancy and maintaining that they are virgins because they have not yet engaged in penile-vaginal sex.<ref name="Intimate"/><ref name="C. Wood">Template:Cite book</ref><ref name="Friedman">Template:Cite news</ref><ref name="Regnerus">Template:Cite book</ref> Likewise, some gay men view frotting or oral sex as maintaining their virginity, with anal penetration regarded as virginity loss, while other gay men consider frotting or oral sex to be their main forms of intercourse.<ref name="Virgin">Template:Cite book</ref><ref name="Underwood">Template:Cite book</ref><ref name="Perez">Template:Cite book</ref><ref name="out.com">Template:Cite news</ref> Lesbians may regard oral sex or fingering as loss of virginity,<ref name="Blank">Template:Cite book</ref><ref name="Bouris">Template:Cite book</ref> and may also regard tribadism as a primary form of sexual activity.<ref name="Weiten, 2008"/><ref name="Greenberg">Template:Cite book</ref>
In 1999, a study by the Kinsey Institute, published in JAMA (the Journal of the American Medical Association), examined the definition of sex based on a 1991 random sample of 599 college students from 29 U.S. states; it reported that 60% said oral-genital contact (fellatio, cunnilingus) did not constitute having sex.<ref name="Cox"/><ref name="Jayson">Template:Cite news</ref> Similarly, a 2003 study published in the Canadian Journal of Human Sexuality focusing on definitions of having sex and noting studies concerning university students from the United States, the United Kingdom, and Australia reported that "[w]hile the vast majority of respondents (more than 97%) in these three studies included penile-vaginal intercourse in their definition of sex, fewer (between 70% and 90%) respondents considered penile-anal intercourse to constitute having sex" and that "oral-genital behaviours were defined as sex by between 32% and 58% of respondents".<ref name="Randall"/> A different study by the Kinsey Institute sampled 484 people, ranging in ages 18–96. "Nearly 95 percent of people in the study agreed that penile-vaginal intercourse meant 'had sex.' But the numbers changed as the questions got more specific." 11 percent of respondents based "had sex" on whether the man had achieved an orgasm, concluding that absence of an orgasm does not constitute "having had" sex. "About 80 percent of respondents said penile-anal intercourse meant 'had sex.' About 70 percent of people believed oral sex was sex."<ref name="Cox"/>
Sexual activity can encompass a number of sexual behaviors, including different sex positions<ref name="Kar"/><ref name="Weiten, 2008"/><ref name="Randall"/> or the use of sex toys. Foreplay may precede certain sexual activities, and often leads to sexual arousal of the partners, resulting in the erection of the penis or (usually) natural lubrication of the vagina.<ref name="Weiten, 2011">Template:Cite book</ref> During coitus, both of the partners move their hips to move the penis backward and forward inside the vagina to cause friction, typically without fully removing the penis. In this way, they stimulate themselves and each other, often continuing until orgasm in either or both partners is achieved.<ref name="Kahn, Fawcett"/> For human females, stimulation of the clitoris plays a significant role in sexual activity; 70–80% require direct clitoral stimulation to achieve orgasm,<ref name="Clitoris">Template:Cite web</ref><ref name="Flaherty">Template:Cite book</ref><ref name="Kenneth Mah">Template:Cite journal</ref><ref name="Kammerer-Doak">Template:Cite journal</ref> though indirect clitoral stimulation (for example, via vaginal intercourse) may also be sufficient (see orgasm in females).<ref name="Masters and Johnson">Template:Cite book</ref><ref name="O'Connell">Template:Cite journal</ref> As such, some couples may engage in the woman on top position or the coital alignment technique, a technique combining the "riding high" variation of the missionary position with pressure-counterpressure movements performed by each partner in rhythm with sexual penetration, to maximize clitoral stimulation.<ref name="Roberts"/><ref name="Kar"/><ref name="Eichel">Template:Cite journal</ref><ref name="Hurlbert">Template:Cite journal</ref>
Anal sex involves stimulation of the anus, anal cavity, sphincter valve or rectum, mostly commonly employing the insertion of a man's penis into another person's rectum.<ref name="Dean and Delvin">Template:Cite web</ref><ref name="Anal Sex">Template:Cite web</ref> Oral sex consists of all the sexual activities that involve the use of the tongue, rest of the mouth and throat to stimulate genitalia. It is sometimes performed to the exclusion of all other forms of sexual activity, and may include the ingestion or absorption of semen or vaginal fluids.<ref name="Kamen"/><ref name="Hite">Template:Cite book</ref> Fingering is the manual (genital) manipulation of the clitoris, vulva, vagina, or anus for the purpose of sexual arousal and sexual stimulation. It may constitute the entire sexual encounter or it may be part of mutual masturbation, foreplay or other sexual activities.<ref name="Hite"/><ref name="Carroll">Template:Cite book </ref>
Bonding and affection
In animals, copulation ranges from a purely reproductive activity to one of emotional bonding between mated pairs. Sexual intercourse and other sexual activity typically plays a powerful role in human bonding. For example, in many societies, it is normal for couples to have frequent intercourse while using some method of birth control (contraception), sharing pleasure and strengthening their emotional bond through sexual activity even though they are deliberately avoiding pregnancy.<ref name="Diamond"/>
In humans and bonobos, the female undergoes relatively concealed ovulation so that both male and female partners commonly do not know whether she is fertile at any given moment. One possible reason for this distinct biological feature may be formation of strong emotional bonds between sexual partners important for social interactions and, in the case of humans, long-term partnership rather than immediate sexual reproduction.<ref name="Diamond"/>
Humans, bonobos, dolphins, and chimpanzees are all intelligent social animals, whose cooperative behavior proves significantly more successful than that of any individual alone. In these animals, the use of sex has evolved beyond reproduction, to apparently serve additional social functions.<ref name=FransDeWaal/><ref name="Bailey"/><ref name="Bagemihl"/> Sex reinforces intimate social bonds between individuals to form larger social structures. The resulting cooperation encourages collective tasks that promote the survival of each member of the group.<ref name="Diamond"/>
Duration and sexual difficulties
Intercourse often ends when the man has ejaculated, and thus the partner might not have time to reach orgasm. In addition, premature ejaculation (PE) is common, and women often require a substantially longer duration of stimulation with a sexual partner than men do before reaching an orgasm.<ref name="Weiten, 2011"/><ref name="Premature ejaculation">Template:Cite web</ref><ref name="Anorgasmia">Template:Cite web</ref> Masters and Johnson found that men took about 4 minutes to reach orgasm with their partners; women took about 10–20 minutes to reach orgasm with their partners, but 4 minutes to reach orgasm when they masturbated.<ref name="Weiten, 2011"/> Scholars Weiten, Dunn and Hammer have reasoned, "Unfortunately, many couples are locked into the idea that orgasms should be achieved only through intercourse [penetrative vaginal sex]. Even the word foreplay suggests that any other form of sexual stimulation is merely preparation for the 'main event.'... ...Because women reach orgasm through intercourse less consistently than men, they are more likely than men to have faked an orgasm."<ref name="Weiten, 2011"/>
In 1991, scholars June M. Reinisch and Ruth Beasley of the Kinsey Institute stated, "The truth is that the time between penetration and ejaculation varies not only from man to man, but from one time to the next for the same man." They added that the appropriate length for intercourse is the length of time it takes for both partners to be mutually satisfied, emphasizing that Kinsey "found that 75 percent of men ejaculated within two minutes of penetration. But he didn't ask if the men or their partners considered two minutes mutually satisfying" and "more recent research reports slightly longer times for intercourse".<ref name="Reinisch">Template:Cite book</ref> A 2008 survey of Canadian and American sex therapists stated that the average time for intromission was 7 minutes and that 1 to 2 minutes was too short, 3 to 7 minutes was adequate and 7 to 13 minutes desirable, while 10 to 30 minutes was too long.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
Anorgasmia is regular difficulty reaching orgasm after ample sexual stimulation, causing personal distress. This is much more common in women than in men.<ref name="Anorgasmia"/><ref name="Frank JE">Template:Cite journal</ref> The physical structure of the act of coitus favors penile stimulation over clitoral stimulation. The location of the clitoris then usually necessitates manual stimulation in order for the female to achieve orgasm.<ref name="Weiten, 2011"/> About 15% of women report difficulties with orgasm, 10% have never climaxed, and 40–50% have either complained about sexual dissatisfaction or experienced difficulty becoming sexually aroused at some point in their lives. 75% of men and 29% of women always have orgasms with their partner.<ref name="Laumann">Template:Cite journal</ref>
Vaginismus is the involuntary tensing of the pelvic floor musculature, making coitus distressing, painful, and sometimes impossible for women.<ref name="Frank JE"/><ref name= Reissing>Reissing ED, Binik YM, Khalifé S, Cohen D, Amsel R. ( 2003) Etiological correlates of vaginismus: sexual and physical abuse, sexual knowledge, sexual self-schema, and relationship adjustment. J Sex Marital Ther.29:47–59.</ref><ref name= Ward>Ward E, Ogden J. (1994) Experiencing Vaginismus: sufferers beliefs about causes and effects. The Journal of Nervous and Mental Disease 9 (1): 33–45.</ref> It is a conditioned reflex of the pubococcygeus muscle, sometimes referred to as the "PC muscle". Vaginismus can be a vicious cycle for women, they expect to experience pain during intercourse, which then causes a muscle spasm, which leads to painful intercourse. Treatment of vaginismus often includes both psychological and behavioral techniques, including the use of vaginal dilators. A new medical treatment using Botox is in the testing phase.<ref name="Frank JE"/><ref>Shafik A.; El-Sibai O.Journal of Obstetrics & Gynaecology, Volume 20, Number 3, 1 May 2000 , pp. 300–302(3)</ref> Some women also experience dyspareunia, a medical term for painful or uncomfortable intercourse, of unknown cause.<ref name=binik2005>Template:Cite journal</ref><ref name=Peckham86>Template:Cite journal</ref>
About 40% of males suffer from some form of erectile dysfunction (ED) or impotence, at least occasionally.<ref>Template:Cite journal</ref> For those whose impotence is caused by medical conditions, prescription drugs such as Viagra, Cialis, and Levitra are available. However, doctors caution against the unnecessary use of these drugs because they are accompanied by serious risks such as increased chance of heart attack. Moreover, using a drug to counteract the symptom—impotence—can mask the underlying problem causing the impotence and does not resolve it. A serious medical condition might be aggravated if left untreated.<ref>Template:Cite journal</ref>
Premature ejaculation is more common than erectile dysfunction. "Estimates vary, but as many as 1 out of 3 men may be affected by [premature ejaculation] at some time."<ref name="Premature ejaculation"/> "Masters and Johnson speculated that premature ejaculation is the most common sexual dysfunction, even though more men seek therapy for erectile difficulties." This is because, "although an estimated 15 percent to 20 percent of men experience difficulty controlling rapid ejaculation, most do not consider it a problem requiring help, and many women have difficulty expressing their sexual needs".<ref name="Reinisch"/> The American Urological Association (AUA) estimates that premature ejaculation could affect 21 percent of men in the United States.<ref>Erectile Dysfunction Guideline Update Panel. AUA guideline on the pharmacologic management of premature ejaculation. Linthicum (MD): American Urological Association, Inc.; 2004. 19 p.</ref> The Food and Drug Administration (FDA or USFDA) is examining the drug dapoxetine to treat premature ejaculation. In clinical trials, those with PE who took dapoxetine experienced intercourse three to four times longer before orgasm than without the drug. Another ejaculation-related disorder is delayed ejaculation, which can be caused as an unwanted side effect of antidepressant medications such as Fluvoxamine.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Although disability-related pain and mobility impairment can hamper intercourse, in many cases the most significant impediments to intercourse for individuals with a disability are psychological.<ref name="Williamson">Template:Cite journal</ref> In particular, people who have a disability can find intercourse daunting due to issues involving their self-concept as a sexual being,<ref name="Majiet">Template:Cite journal</ref><ref name="Dewolfe">Template:Cite journal</ref> or partner's discomfort or perceived discomfort.<ref name="Williamson" /> Temporary difficulties can arise with alcohol and sex as alcohol initially increases interest (through disinhibition) but decreases capacity with greater intake.<ref name="LC2">Template:Cite journal</ref>
Reproduction, reproductive methods and pregnancy
Reproduction among humans usually occurs with penile-vaginal penetration.<ref name="Evan Jones">Template:Cite book</ref> Male orgasm usually includes ejaculation, a series of muscular contractions that deliver semen containing male gametes known as sperm cells or spermatozoa from the penis into the vagina. The subsequent route of the sperm from the vault of the vagina is through the cervix and into the uterus, and then into the fallopian tubes. Millions of sperm are present in each ejaculation, to increase the chances of one fertilizing an egg or ovum (see sperm competition). When a fertile ovum from the female is present in the fallopian tubes, the male gamete joins with the ovum, resulting in fertilization and the formation of a new embryo. When a fertilized ovum reaches the uterus, it becomes implanted in the lining of the uterus – known as the endometrium – and a pregnancy begins.<ref name="Evan Jones"/> Unlike most species, human sexual activity is not linked to periods of estrus and can take place at any time during the reproductive cycle, even during pregnancy.<ref>Template:Cite book</ref> Where a sperm donor has sexual intercourse with a woman who is not his partner, for the sole purpose of impregnating the woman, this may be known as natural insemination, as opposed to artificial insemination. However, most sperm donors donate their sperm through a sperm bank and pregnancy is achieved through artificial insemination.<ref>Template:Cite web</ref>
In 2005, the World Health Organization (WHO) estimated that 123 million women become pregnant world-wide each year, and around 87 million of those pregnancies or 70.7% are unintentional. About 46 million pregnancies per year reportedly end in induced abortion.<ref>Template:Cite web</ref> About 6 million U.S. women become pregnant per year. Out of known pregnancies, two-thirds result in live births and roughly 25% in abortions; the remainder end in miscarriage. However, many more women become pregnant and miscarry without even realizing it, instead mistaking the miscarriage for an unusually heavy menstruation.<ref name="Guttmacher 0">Template:Cite web</ref> The U.S. teenage pregnancy rate fell by 27 percent between 1990 and 2000, from 116.3 pregnancies per 1,000 girls aged 15–19 to 84.5. This data includes live births, abortions, and fetal losses. Almost 1 million American teenage women, 10% of all women aged 15–19 and 19% of those who report having had intercourse, become pregnant each year.<ref name="Ventura">Template:Cite web</ref> Britain has been stated to have a teenage pregnancy rate similar to America's.<ref name="British pregnancy">Template:Cite web</ref>
Reproductive methods and pregnancy also extend to gay and lesbian couples. For gay male pairings, there is the option of surrogate pregnancy; for lesbian couples, there is donor insemination in addition to choosing surrogate pregnancy.<ref name="Berkowitz">Berkowitz, D & Marsiglio, W (2007). Gay Men: Negotiating Procreative, Father, and Family Identities. Journal of Marriage and Family 69 (May 2007): 366–381</ref><ref name="Burda"> Template:Cite book</ref> Further, developmental biologists have been researching and developing techniques to facilitate biological same-sex reproduction, though this has yet to be demonstrated in humans (see same-sex reproduction).<ref name="chicken sperm">Template:Cite journal</ref><ref>Template:Cite news</ref> Surrogacy and donor insemination remain the primary methods. Surrogacy is an arrangement in which a woman carries and delivers a child for another couple or person. The woman may be the child's genetic mother (traditional surrogacy) or she may carry a pregnancy to delivery after having another woman's eggs transferred to her uterus (gestational surrogacy). Gay and lesbian pairings who want the host to have no genetic connection to the child may choose gestational surrogacy and enter into a contract with an egg donor. Gay male couples might decide that they should both contribute semen for the in vitro fertilisation (IVF) process, which further establishes the couple's joint intention to become parents.<ref name="Burda"/> Lesbian couples often have contracts drafted to extinguish the legal rights of the sperm donor, while creating legal rights for the parent who is not biologically related to the child.<ref name="Schaffner">Template:Cite book</ref>
Safe sex, contraception, and prevalence of sexual activity
Template:See also Template:Globalize There are a variety of safe sex practices, including non-penetrative sex acts,<ref name="Durham"/><ref name="aids.gov"/> and heterosexual couples may use oral or anal sex (or both) as a means of birth control (contraception).<ref name="C. Wood"/><ref name="Regnerus"/> Safe sex is a relevant harm reduction philosophy.<ref name="Who"/><ref>Template:Cite web</ref> Condoms are used as a form of safe sex and as a form of contraception. Condoms and dental dams are widely recommended for the prevention of sexually transmitted infections (STIs). According to reports by the National Institutes of Health (NIH) and World Health Organization (WHO), correct and consistent use of latex condoms reduces the risk of HIV/AIDS transmission by approximately 85–99% relative to risk when unprotected.<ref>Template:Cite conference</ref><ref>World Health Organization. Effectiveness of male latex condoms in protecting against pregnancy and sexually transmitted infections , 2000.</ref>
According to a national survey conducted in the U.S. in 1995, at least 3/4 of all men and women in the U.S. have had intercourse by their late teenage years, and more than 2/3 of all sexually experienced teens have had 2 or more partners.<ref name="Guttmacher 1">Template:Cite web</ref> In 2004, the Guttmacher Institute indicated in 2002 that 62% of the 62 million women aged 15–44 are currently using a contraceptive method, that among U.S. women who practice contraception, the Pill is the most popular choice (30.6%), followed by tubal sterilization (27.0%) and the male condom (18.0%), and that 27% of teenage women using contraceptives choose condoms as their primary method.<ref name="Guttmacher 2">Template:Cite web</ref> A 2006 Kaiser Family Foundation report stated that among sexually active 15- to 19-year-olds in the U.S., 83% of females and 91% of males reported using at least one method of birth control during last intercourse.<ref name="KFF_factsheet">Template:Cite web</ref>
A 2006 survey conducted by The Observer suggested that most adolescents in Britain were waiting longer to have sexual intercourse than they were only a few years earlier.<ref name="British sex">Template:Cite journal</ref> In 2002, it was reported that 32% of British teenagers were having sex before the age of 16, while, in 2006, it was only 20%. The average age a British teenager lost his/her virginity was reportedly 17.13 years in 2002; in 2006, it was 17.44 years on average for girls and 18.06 for boys. The most notable drop among teens who reported having sex was 14- and 15-year-olds.<ref name="British sex"/> A 2008 survey conducted by YouGov for Channel 4 suggested that 40% of all 14- to 17-year-olds are sexually active, 74% of sexually active 14- to 17-year-olds have had a sexual experience under the age of consent, and 6% of teens would wait until marriage before having sex.<ref name="sex stats">Template:Cite web</ref>
The National Survey of Sexual Health and Behavior (NSSHB) indicated in 2010 that 1 of 4 acts of vaginal intercourse are condom-protected in the U.S. (1 in 3 among singles), condom use is higher among black and Hispanic Americans than among white Americans and those from other racial groups, and adults using a condom for intercourse were just as likely to rate the sexual extent positively in terms of arousal, pleasure and orgasm than when having intercourse without one.<ref name="NSSHB">Template:Cite journal</ref>
In humans, sexual activity has been claimed to produce health benefits as varied as improved sense of smell,<ref>Wood, H. Sex Cells Nature Reviews Neuroscience 4, 88 (February 2003) | Template:Doi</ref> stress and blood pressure reduction,<ref name="webmd.com">Doheny, K. (2008) "10 Surprising Health Benefits of Sex," WebMD (reviewed by Chang, L., M.D.)</ref><ref>Light, K.C. et al., "More frequent partner hugs and higher oxytocin levels are linked to lower blood pressure and heart rate in premenopausal women." Biological Psychology, April 2005; vol 69: pp 5–21.</ref> increased immunity,<ref>Charnetski CJ, Brennan FX. Sexual frequency and salivary immunoglobulin A (IgA). Psychological Reports 2004 Jun;94(3 Pt 1):839-44. Data on length of relationship and sexual satisfaction were not related to the group differences.</ref> and decreased risk of prostate cancer.<ref>Michael F. Leitzmann; Edward Giovannucci. Frequency of Ejaculation and Risk of Prostate Cancer—Reply. JAMA. (2004);292:329.</ref><ref>Leitzmann MF, Platz EA, Stampfer MJ, Willett WC, Giovannucci E. Ejaculation Frequency and Subsequent Risk of Prostate Cancer. JAMA. (2004);291(13):1578–1586.</ref><ref>Template:Cite journal</ref> Sexual intimacy, as well as orgasms, increases levels of the hormone oxytocin, also known as "the love hormone", which helps people bond and build trust.<ref>Template:Cite journal</ref><ref>Riley AJ. Oxytocin and coitus. Sexual and Relationship Therapy (1988);3:29–36</ref><ref>Carter CS. Oxytocin and sexual behavior. Neuroscience & Biobehavioral Reviews (1992);16(2):131–144</ref> Sex is also known as one of many mood repair strategies, which means it can be used to help dissipate feelings of sadness or depression.<ref name="Thayer">Thayer, R. E., Newman, J., & McClain, T. M. (1994). Self-regulation of mood: Strategies for changing a bad mood, raising energy, and reducing tension. Journal Of Personality And Social Psychology, 67(5), 910–925.</ref> A long-term study of 3,500 people between 30 and 101 by clinical neuropsychologist David Weeks, MD, head of old age psychology at the Royal Edinburgh Hospital in Scotland, found that "sex helps you look between four and seven years younger", according to impartial ratings of the subjects' photos. Exclusive causation, however, is unclear, and the benefits may be indirectly related to sex and directly related to significant reductions in stress, greater contentment, and better sleep that sex promotes.<ref>Template:Cite web</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref>
Sexually transmitted infections (STIs) can be spread by person-to-person sexual contact, including sexual intercourse. There are 19 million new cases of sexually transmitted diseases every year in the U.S.,<ref>Template:Cite web</ref> and, in 2005, the World Health Organization estimated that 448 million people aged 15–49 were being infected a year with curable STIs such as syphilis, gonorrhoea and chlamydia.<ref name=WHO-2011>Template:Cite web</ref> In 2006, The Independent newspaper reported that the biggest rise in sexually transmitted infections was in syphilis, which rose by more than 20%, while increases were also seen in cases of genital warts and herpes.<ref name="sti">Template:Cite journal</ref>
STIs are caused by bacteria, viruses and parasites, which are passed from person to person during sexual contact. Some, in particular HIV and syphilis, can also be passed in other ways including from mother to child during pregnancy and childbirth, through blood products, and by shared hypodermic needles.<ref name=WHO-2011/> Gonococcal or chlamydial infections often produce no symptoms at all. Untreated chlamydial infection can lead to female infertility and ectopic pregnancy. Human papillomavirus can lead to genital and cervical cancers. Syphilis can result in stillbirths and neonatal deaths. Untreated gonococcal infections result in miscarriages, preterm births, and perinatal deaths. Infants born to mothers with untreated gonorrhoea or chlamydia can develop serious eye infections, which can lead to blindness.<ref name=WHO-2011/> Hepatitis B can also be transmitted through sexual contact.<ref>CDC Hepatitis B Information for Health Professionals Accessed May 27, 2010</ref> Globally, there are about 350 million chronic carriers of hepatitis B.<ref>Template:Cite web</ref>
Some STIs can cause ulceration, but even if they do not, they increase the risk of both acquiring and passing on HIV up to ten-fold.<ref name=WHO-2011/> HIV is one of the world's leading infectious killers, and, in 2010, approximately 30 million people were estimated to have died because of it since the beginning of the epidemic. Of the 2.7 million new HIV infections estimated to occur worldwide in 2010, 1.9 million (70%) were in Africa. "The estimated 1.2 million Africans who died of HIV-related illnesses in 2010 comprised 69% of the global total of 1.8 million deaths attributable to the epidemic."<ref name=WHO-HIV>Template:Cite web</ref> It is diagnosed by blood tests, and while no cure has been found, it can be controlled with antiretroviral drugs, and patients can enjoy healthy and productive lives.<ref name=WHO-2012>Template:Cite web</ref>
The most effective way to avoid sexually transmitted infections is to abstain from sexual intercourse, including oral, vaginal, and anal sex, or to have sexual intercourse only with one long-term, uninfected partner, who also remains entirely monogamous. The World Health Organization says that "Male latex condoms, when used consistently and correctly, are highly effective in reducing the transmission of HIV and other sexually transmitted infections, including gonorrhoea, chlamydial infection and trichomoniasis."<ref name=WHO-2011/> In cases where infection is suspected, early medical intervention is highly beneficial in all cases.
People, especially those who get little or no physical exercise, have a slightly increased risk of triggering heart attack or sudden cardiac death when they engage in sexual intercourse, or any other vigorous physical exercise which is engaged in on a sporadic basis. Increased risk is temporary with incidents occurring within a few hours of the activity. Regular exercise reduces but does not eliminate the increased risk.<ref>Template:Cite journal</ref>
Alex Comfort and others posit three potential advantages of intercourse in humans, which are not mutually exclusive: reproductive, relational, and recreational.<ref name="Diamond"/><ref name="Comfort">The Joy of Sex: A Gourmet Guide to Lovemaking (1972)</ref> While the development of the Pill and other highly effective forms of contraception in the mid- and late 20th century increased people's ability to segregate these three functions, they still overlap a great deal and in complex patterns. For example: A fertile couple may have intercourse while contracepting not only to experience sexual pleasure (recreational), but also as a means of emotional intimacy (relational), thus deepening their bonding, making their relationship more stable and more capable of sustaining children in the future (deferred reproductive). This same couple may emphasize different aspects of intercourse on different occasions, being playful during one episode of intercourse (recreational), experiencing deep emotional connection on another occasion (relational), and later, after discontinuing contraception, seeking to achieve pregnancy (reproductive, or more likely reproductive and relational).<ref name="Comfort"/>
Nearly all Americans marry during their lifetime; yet close to half of all first marriages are expected to end in separation or divorce, many within a few years,<ref name="Bramlett">Bramlett, M. D. and W. D. Mosher (2002). "Cohabitation, marriage, divorce, and remarriage in the United States." Vital & Health Statistics – Series 23, Data from the National Survey of Family Growth 22: 1–93.</ref> and subsequent marriages are even more likely to end.<ref name="Karney">Template:Cite journal</ref> Sexual dissatisfaction is associated with increased risk of divorce and relationship dissolution.<ref name="Karney"/>
According to the National Survey of Sexual Health and Behavior (NSSHB), in 2010, men whose most recent sexual encounter was with a relationship partner reported greater arousal, greater pleasure, fewer problems with erectile function, orgasm, and less pain during the event than men whose last sexual encounter was with a non-relationship partner.<ref name="NSSHB"/> According to the Journal of Counseling & Development, many women express that their most satisfying sexual experiences entail being connected to someone, rather than solely basing satisfaction on orgasm.<ref name="Bridges">Template:Cite journal</ref>
With regard to adolescent sexuality, sexual intercourse is often for relational and recreational purposes as well. However, teenage pregnancy is usually disparaged, and research suggests that the earlier onset of puberty for children puts pressure on children and teenagers to act like adults before they are emotionally or cognitively ready,<ref name="sex lives3">Template:Cite book</ref><ref name="Garn">Garn, SM. Physical growth and development. In: Friedman SB, Fisher M, Schonberg SK. , editors. Comprehensive Adolescent Health Care. St Louis: Quality Medical Publishing; 1992. Retrieved on 2009-02-20</ref> and thus are at risk to suffer from emotional distress as a result of their sexual activities.<ref name="Garn"/><ref name="USATodayTeens">Template:Cite news</ref><ref name="oconnell">Template:Cite news</ref><ref>(Caspi et al.1993: Lanza and Collins, 2002)</ref><ref>(Stattin & Magnussion, 1990).</ref> Some studies have concluded that engaging in sex leaves adolescents, and especially girls, with higher levels of stress and depression.<ref name="depress">Template:Cite journal</ref> A majority of adolescents in the United States have been provided with some information regarding sexuality,<ref name="Couric">Template:Cite web</ref> though there have been efforts among social conservatives in the United States government to limit sex education in public schools to abstinence-only sex education curricula.<ref name="Kaiser 2002">Template:Cite web</ref>
One group of Canadian researchers found a relationship between self-esteem and sexual activity. They found that students, especially girls, who were verbally abused by teachers or rejected by their peers were more likely than other students to engage in sex by the end of the Grade 7. The researchers speculate that low self-esteem increases the likelihood of sexual activity: "low self-esteem seemed to explain the link between peer rejection and early sex. Girls with a poor self-image may see sex as a way to become 'popular', according to the researchers".<ref name="Peer rejection">Template:Cite web</ref>
In India, there is growing evidence that adolescents are becoming more sexually active outside of marriage, which is feared to lead to an increase in the spread of HIV/AIDS among adolescents, as well as the number of unwanted pregnancies and abortions, and add to the conflict between contemporary social values. In India, adolescents have relatively poor access to health care and education, and with cultural norms opposing extramarital sexual behavior, "these implications may acquire threatening dimensions for the society and the nation".<ref name="india girls">Template:Cite web</ref>
Not all views on adolescent sexual behavior are negative, however. Psychiatrist Lynn Ponton writes, "All adolescents have sex lives, whether they are sexually active with others, with themselves, or seemingly not at all," and that viewing adolescent sexuality as a potentially positive experience, rather than as something inherently dangerous, may help young people develop healthier patterns and make more positive choices regarding sex.<ref name="sex lives3">Template:Cite book</ref> Likewise, others state that long-term romantic relationships allow adolescents to gain the skills necessary for high-quality relationships later in life<ref>Madsen S., Collins W. A. (2005). Differential predictions of young adult romantic relationships from transitory vs. longer romantic experiences during adolescence. Presented at Biennial Meeting of the Society for Research on Child Development, Atlanta, GA.</ref> and develop feelings of self-worth. Overall, positive romantic relationships among adolescents can result in long-term benefits. High-quality romantic relationships are associated with higher commitment in early adulthood<ref>Seiffge-Krenke I., Lang J. (2002). Forming and maintaining romantic relations from early adolescence to young adulthood: evidence of a developmental sequence. Presented at Biennial Meeting of the Society for Research on Adolescence, 19th, New Orleans, LA.</ref> and are positively associated with self-esteem, self-confidence, and social competence.<ref>Pearce M. J., Boergers J., Prinstein M.J., (2002). Adolescent obesity, overt and relational peer victimization, and romantic relationships. Obesity Research, 10, 386–93.</ref><ref>Zimmer-Gembeck, M.J., Siebenbruner, J., & Collins, W.A. (2004). A prospective study of intraindividual and peer influences on adolescents’ heterosexual romantic and sexual behavior. Archives of Sexual Behavior, 33, 381–394.</ref>
Ethical, religious, and legal views
While sexual intercourse is the natural mode of reproduction for the human species, humans have intricate moral and ethical guidelines which regulate the practice of sexual intercourse and vary according to religious and governmental laws. Some governments and religions also have strict designations of "appropriate" and "inappropriate" sexual behavior, which include restrictions on the types of sex acts which are permissible. A historically prohibited or regulated sex act is anal sex.<ref>William N. Eskridge Jr. Dishonorable Passions: Sodomy Laws in America, 1861–2003. (2008) Viking Adult. ISBN 0-670-01862-7</ref><ref>Noelle N. R. Quenivet. Sexual Offenses in Armed Conflict & International Law. (2005) Hotei Publishing. ISBN 1-57105-341-7</ref>
Consent and sexual offenses
Sexual intercourse with a person against their will, or without their informed legal consent, is rape, and is considered a serious crime in most countries.<ref>Marshall Cavendish Corporation Staff. Sex and Society. (2009) Cavendish, Marshall Corporation. p143-144. ISBN 0-7614-7906-6 </ref> More than 90% of rape victims are female, 99% of rapists male, and only about 5% of rapists are strangers to the victims.<ref>Jerrold S. Greenberg, Clint E. Bruess, Sarah C. Conklin. (2010). Exploring the Dimensions of Human Sexuality. p 515. ISBN 0-7637-7660-2 .</ref>
Most developed countries have age of consent laws specifying the minimum legal age a person may engage in sexual intercourse with substantially older persons, usually set at about 16–18, while the legal age of consent ranges from 12–20 years of age or is not a matter of law in other countries.<ref>Karen L. Kinnear. (2007) Childhood sexual abuse: a reference handbook. ABC-CLIO. ISBN 1-85109-905-0 </ref> Sex with a person under the age of consent, regardless of their stated consent, is often considered to be sexual assault or statutory rape depending on differences in ages of the participants.
Some countries codify rape as any sex with a person of diminished or insufficient mental capacity to give consent, regardless of age.<ref>Template:Cite journal</ref>
The expression "sexual intercourse" has been used as a term of art in England and Wales and New York State. In England and Wales, from its enactment to its repeal on the 1 May 2004,<ref>The Sexual Offences Act 2003 (Commencement) Order 2004 (S.I. 2004/874)</ref> section 44 of the Sexual Offences Act 1956 read:
This expression refers to buggery, including both buggery with a person and buggery with an animal.<ref>R v Gaston 73 Cr App R 164, CA</ref> Zoophilia (bestiality) is sexual activity between humans and non-human animals or a preference for or fixation on such practice. People who practice zoophilia are known as zoophiles,<ref name=EB>"Zoophilia," Encyclopaedia Britannica, 2009; Retrieved 24 January 2009.</ref> zoosexuals, or simply "zoos".<ref name="Handbookth">Template:Cite journal</ref> Zoophilia may also be known as zoosexuality.<ref name="Handbookth"/> Zoophilia is a paraphilia.<ref name=DSM>Template:Cite book</ref><ref name = Milner2008>Template:Cite book</ref><ref name = Lovemaps>Template:Cite book</ref><ref name = Seto2000>Template:Cite book</ref> Sex with animals is not outlawed in some jurisdictions, but, in most countries, it is illegal under animal abuse laws or laws dealing with crimes against nature.
According to cases decided on the meaning of the statutory definition of carnal knowledge under the Offences against the Person Act 1828, which was in identical terms to this definition, the slightest penetration was sufficient.<ref>R v R'Rue (1838) 8 C & P 641; R v Allen (1839) 9 C & P 31</ref> The book "Archbold" said that it "submitted" that this continued to be the law under the new enactment.<ref>Archbold Criminal Pleading, Evidence and Practice, 1999, paragraph 20–24</ref>
- Continuing act
See Kaitamaki v R  AC 147,  3 WLR 137,  2 All ER 435, 79 Cr App R 251,  Crim LR 564, PC (decided under equivalent legislation in New Zealand).
Section 7(2) of the Sexual Offences (Amendment) Act 1976 contained the following words: "In this Act . . . references to sexual intercourse shall be construed in accordance with section 44 of the Sexual Offences Act 1956 so far as it relates to natural intercourse (under which such intercourse is deemed complete on proof of penetration only)". The Act made provision, in relation to rape and related offences, for England and Wales, and for courts-martial elsewhere.
From 3 November 1994 to 1 May 2004, section 1(2)(a) of the Sexual Offences Act 1956 (as substituted by section 142 of the Criminal Justice and Public Order Act 1994) referred to "sexual intercourse with a person (whether vaginal or anal)". This section created the offence of rape in England and Wales.
The penal code in New York State provides: § 130.00 Sex offenses; definitions of terms: 1. "Sexual intercourse" has its ordinary meaning and occurs upon any penetration, however slight.<ref>Template:Cite web</ref>
Sexual orientation and gender
There is considerable legal variability regarding definitions of and the legality of sexual intercourse between persons of the same sex or gender. For example, in 2003 the New Hampshire Supreme Court ruled that female same-sex relations did not constitute sexual intercourse, based on a 1961 definition from Webster's Third New International Dictionary, in Blanchflower v. Blanchflower, and thereby an accused wife in a divorce case was found not guilty of adultery based on this technicality. Some countries, such as Islamic countries, consider homosexual behavior to be an offense punishable by imprisonment or execution.<ref>Janet Afary. Sexual Politics in Modern Iran. (2009) Cambridge University Press. ISBN 0-521-89846-3</ref>
Marriage and relationships
Sexual intercourse has traditionally been considered an essential part of a marriage; many religious customs required consummation of the marriage by sexual intercourse, and the failure for any reason to consummate the marriage was a ground for annulment, which did not require a divorce process. Annulment declaration implied that the marriage was void from the start – i.e. there was in law no marriage. Furthermore, continuing sexual relations between the marriage partners is commonly considered a 'marital right' by many religions, permissible to married couples, generally for the purpose of reproduction. Today, there is wide variation in the opinions and teachings about sexual intercourse relative to marriage and other intimate relationships by the world's religions. Examples:
- Most denominations of Christianity, including Catholicism,<ref name=Akin/> have strict views or rules on what sexual practices are acceptable or, more specifically, what are not.<ref name=ChristianView1>Template:Cite web</ref> Most Christian views on sex are formed or influenced by various interpretations of the Bible.<ref>Template:Cite web</ref> Sex outside of marriage is considered a sin in some churches, and sex may be referred to as a "sacred covenant" between husband and wife. Historically, Christian teachings often promoted celibacy,<ref name=Family>Template:Cite web</ref> although today usually only certain members (for example certain religious leaders) of some groups take a vow of celibacy, forsaking both marriage and any type of sexual or romantic activity. Some Christians view sex, particularly sexual intercourse between a married couple, as "holy" or a "holy sacrament".<ref name=Akin /><ref name=Family /> Some Christians interpret the Bible to forbid the "misuse of sexual organs" and take that to mean that only penile/vaginal penetrative intercourse is acceptable, while some argue that the Bible is not clear on oral sex and that it is a personal decision as to whether its acceptable within marriage.<ref name=Taylor>Template:Cite web</ref>
- In The Church of Jesus Christ of Latter-day Saints, or Mormonism, sexual relations within the bonds of matrimony are seen as beautiful and sacred. Mormons consider sexual relations to be ordained of God for the creation of children and for the expression of love between husband and wife. Members are encouraged to not have any sexual relations before marriage, and be completely faithful to their spouse after marriage.<ref>For the Strength of Youth: Sexual Purity</ref>
- In Judaism, a married Jewish man is required to provide his wife with sexual pleasure called onah (literally, "her time"), which is one of the conditions he takes upon himself as part of the Jewish marriage contract, ketubah, that he gives her during the Jewish wedding ceremony. In Jewish views on marriage, sexual desire is not evil, but must be satisfied in the proper time, place and manner.<ref>Template:Cite web</ref>
- Islam views sex within marriage as something pleasurable, a spiritual activity, and a duty.<ref name=columbia/><ref name=islam>Abdul Rahman bin Abdul Karim al-Sheha. Islamic Perspective of Sex (2003) Saudi Arabia. ISBN 9960-43-140-1</ref><ref name=islam2>Fatima M. D'Oyen. The Miracle of Life. (2007)Islamic Foundation (UK). ISBN 0-86037-355-X</ref> In Shi'ia Islam, men are allowed to enter into an unlimited number of temporary marriages, which are contracted to last for a period of minutes to multiple years and permit sexual intercourse. Shi'ia women are allowed to enter only one marriage at a time, whether temporary or permanent.
- Wiccans believe that, as declared within the Charge of the Goddess, to "Let my [the Goddess] worship be within the heart that rejoiceth; for behold, all acts of love and pleasure are my rituals." This statement appears to allow one freedom to explore sensuality and pleasure, and mixed with the final maxim within the Wiccan Rede – "26. Eight words the Wiccan Rede fulfill – an’ it harm none, do what ye will."<ref>Thompson, Lady Gwen; Wiccan-Pagan Potpourri; Green Egg, №69; Ostara 1974</ref> – Wiccans are encouraged to be responsible with their sexual encounters, in whatever variety they may occur.<ref>Hans Holzer. The Truth about Witchcraft (1971) Doubleday. page 128. ISBN 0-09-004860-1</ref>
- Hinduism has varied views about sexuality, but Hindu society, in general, perceives extramarital sex to be immoral and shameful.<ref name=columbia>Don S. Browning, Martha Christian Green, John Witte. Sex, marriage, and family in world religions. (2006) Columbia University Press. ISBN 0-231-13116-X </ref>
- Buddhist ethics, in its most common formulation, holds that one should neither be attached to nor crave sensual pleasure.
- In the Bahá'í Faith, sexual relationships are permitted only between a husband and wife.<ref>Kenneth E. Bowers. God Speaks Again: An Introduction to the Baha'i Faith. (2004) Baha'i Publishing. ISBN 1-931847-12-6</ref>
- Unitarian Universalists, with an emphasis on strong interpersonal ethics, do not place boundaries on the occurrence of sexual intercourse among consenting adults.<ref>John A. Buehrens and Forrest Church. A Chosen Faith: An Introduction to Unitarian Universalism. (1998) Beacon Press. ISBN 0-8070-1617-9</ref>
- According to the Brahma Kumaris and Prajapita Brahma Kumaris religion, the power of lust is the root of all evil and worse than murder.<ref name="Liz_Hodgkinson_Peace">Template:Cite book</ref> Purity (celibacy) is promoted for peace and to prepare for life in forthcoming Heaven on earth for 2,500 years when children will be created by the power of the mind.<ref>Babb, Lawrence A. (1987). Redemptive Encounters: Three Modern Styles in the Hindu Tradition (Comparative Studies in Religion and Society). Oxford University Press. ISBN 0-7069-2563-7. "Sexual intercourse is unnecessary for reproduction because the souls that enter the world during the first half of the Cycle are in possession of a special yogic power (yog bal) by which they conceive children"</ref><ref>Barrett, David V (2001). The New Believers. Cassell & Co. pp. 265. ISBN 0-304-35592-5.</ref>
- Shakers believe that sexual intercourse is the root of all sin and that all people should therefore be celibate, including married couples. Predictably, the original Shaker community that peaked at 6,000 full members in 1840 dwindled to three members by 2009.<ref>Template:Cite news</ref>
In some cases, the sexual intercourse between two people is seen as counter to religious law or doctrine. In many religious communities, including the Catholic Church and Mahayana Buddhists, religious leaders are expected to refrain from sexual intercourse in order to devote their full attention, energy, and loyalty to their religious duties.<ref>William Skudlarek. Demythologizing Celibacy: Practical Wisdom from Christian and Buddhist Monasticism. (2008) Liturgical Press. ISBN 0-8146-2947-4</ref>
Opposition to same-sex marriage is largely based on the belief that sexual intercourse and sexual orientation should be of a heterosexual nature.<ref name="Cahn and Carbone">Template:Cite book</ref><ref name="Cantor">Template:Cite book</ref><ref name="psychological">Template:Cite web</ref><ref name="psychiatric">Template:Cite web</ref> The recognition of such marriages is a civil rights, political, social, moral, and religious issue in many nations, and the conflicts arise over whether same-sex couples should be allowed to enter into marriage, be required to use a different status (such as a civil union, which either grant equal rights as marriage or limited rights in comparison to marriage), or not have any such rights. A related issue is whether the term marriage should be applied.<ref name="Taylor">Template:Cite news</ref><ref name="Smith">Template:Cite news</ref><ref name="Vs.">Template:Cite web</ref>
In other animals
In zoology, copulation is often termed as the process in which a male introduces sperm into the female's body. Spiders have separate male and female sexes. Before mating and copulation, a male spins a small web and ejaculates on to it. He then stores the sperm in reservoirs on his large pedipalps, from which he transfers sperm to the female's genitals. Females can store sperm indefinitely.<ref name="RFB2004Spiders">Template:Cite book</ref>
Many animals which live in the water use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch. Internal fertilization with many vertebrates (such as reptiles, some fish, and most birds) occur via cloacal copulation (see also hemipenis), while mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.
However, some terrestrial arthropods do use external fertilization. For primitive insects, the male deposits spermatozoa on the substrate, sometimes stored within a special structure, and courtship involves inducing the female to take up the sperm package into her genital opening; there is no actual copulation. In groups such as dragonflies and spiders, males extrude sperm into secondary copulatory structures removed from their genital opening, which are then used to inseminate the female (in dragonflies, it is a set of modified sternites on the second abdominal segment; in spiders, it is the male pedipalps). In advanced groups of insects, the male uses its aedeagus, a structure formed from the terminal segments of the abdomen, to deposit sperm directly (though sometimes in a capsule called a "spermatophore") into the female's reproductive tract.
Humans, bonobos,<ref name = "FransDeWaal">Frans de Waal, "Bonobo Sex and Society", Scientific American (March 1995): 82–86.</ref> chimpanzees and dolphins<ref name=Bailey/> are species known to engage in heterosexual behaviors even when the female is not in estrus, which is a point in her reproductive cycle suitable for successful impregnation. These species, and others, are also known to engage in homosexual behaviors.<ref name=Bagemihl/> Humans, bonobos and dolphins are all intelligent social animals, whose cooperative behavior proves far more successful than that of any individual alone. In these animals, the use of sex has evolved beyond reproduction, to apparently serve additional social functions. Sex reinforces intimate social bonds between individuals to form larger social structures. The resulting cooperation encourages collective tasks that promote the survival of each member of the group.<ref name="Diamond"/>
- The International Encyclopedia of Sexuality
- Glossario di Sessuologia clinica – Glossary of clinical sexology
- Janssen, D. F., Growing Up Sexually. Volume I. World Reference Atlas
- Introduction to Animal Reproduction
- Advantages of Sexual Reproduction
- Synonyms for sexual intercourse – the WikiSaurus list of synonyms and slang words for sexual intercourse in many languages
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