Sexual intercourse, also known as copulation or coitus, commonly refers to the act in which the male reproductive organ enters the female reproductive tract.  The two entities may be of opposite sexes or not, or they may be hermaphroditic, as is the case with snails. In recent years, penetration of non-sexual organs (oral intercourse, anal intercourse) or by non-sexual organs are also sometimes included in this definition. Traditionally, intercourse has been viewed as the natural endpoint of all sexual contact between a man and a woman.
Non-penetrative sexual contacts and mutual masturbation are usually referred to as outercourse (oral sex may or not be penetrative),
Sexual intercourse for most animals other than humans occurs at the point of estrus (the most fertile period of time in the female's reproductive cycle),"Females of almost all species except man will mate only during their fertile period, which is known as estrus, or heat..." However, in most instances, humans have sex primarily for pleasure. This behavior in the above mentioned animals is also presumed to be for pleasure, which in turn strengthens social bonds.
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, 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 many 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.
Vaginal sexual intercourse, also called coitus, is the human form of copulation. While a purpose and effect is reproduction, it is often performed exclusively for pleasure and/or as an expression of love and emotional intimacy. Sexual intercourse typically plays a powerful bonding role; in many societies it is normal for couples to have frequent intercourse while using birth control, sharing pleasure and strengthening their emotional bond through sex even though they are deliberately avoiding pregnancy.
Sexual intercourse may also be defined as referring to other forms of insertive sexual behavior, such as oral sex and anal intercourse. The phrase to have sex can mean any or all of these behaviors, as well as other non-penetrative sex acts not considered here.
To engage in coitus, the erect penis is inserted into the vagina and one or 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. Penetration by the hardened erect penis is also known as intromission, or by the Latin name immissio (Latin for "insertion of the penis").
Coitus is the basic reproductive method of humans. During ejaculation, which usually accompanies male orgasm, a series of muscular contractions delivers 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. 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 endometrium and a pregnancy begins.
Unlike most species, most human copulation takes place in private. Furthermore, human sexual activity is not linked to periods of estrus and can take place at any time during the reproductive cycle, even during pregnancy.
Anorgasmia is regular difficulty reaching orgasm after ample sexual stimulation, causing personal distress. The physical structure of the act of coitus favors penile stimulation over clitoral stimulation. The location of the clitoris then often necessitates manual stimulation in order for the female to achieve orgasm. About 15 percent of women report difficulties with orgasm, and as many as 10 percent of women in the United States have never climaxed. Even women who orgasm regularly only climax about 50 percent to 70 percent of the time. 
Some males suffer from erectile dysfunction (ED), or impotence, at least occasionally. 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.
A more common sexual disorder in males is premature ejaculation (PE). The U.S. Food and Drug Administration 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.
The American Urological Association (AUA) estimates that premature ejaculation could affect 27 to 34 percent of men in the United States. The AUA also estimates that 10 to 12 percent of men in the United States are affected by erectile dysfunction. Vaginismus is involuntary tensing of the pelvic floor musculature, making coitus distressing, painful, and sometimes impossible. Dyspareunia is a medical term signifying painful or uncomfortable intercourse, but does not specify the cause.
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. In particular, people who have a disability can find intercourse daunting due to issues involving their self-concept as a sexual being,  or partner's discomfort or perceived discomfort.
Functions of sex beyond reproduction
Sex as exercise burns calories to produce health benefits. Sex relieves stress, boosts the immune system with higher levels of immunoglobulin A, improves cardiovascular health, increases self-esteem, improves intimacy, reduces pain by production of the hormone oxytocin, reduces the risk of prostate cancer, strengthens pelvic muscles, and promotes good sleep. In addition, sex improves the sense of smell and urinary bladder control.
According to researchers from Scotland who reported their findings in the journal Biological Psychology, a significant health benefit of sex is lower blood pressure and overall stress reduction. They studied 24 women and 22 men who kept records of their sexual activity. The researchers subjected the men and women to stressful situations, such as speaking in public and doing verbal arithmetic, and noted their blood pressure response to stress. The men and women who had intercourse responded more positively to stress "than those who engaged in other sexual behaviors or abstained". Researchers found that another study published in the same journal documented frequent intercourse as being associated with lower diastolic blood pressure in cohabiting participants, though other research "found a link between partner hugs and lower blood pressure in women".
People who have a "good and healthy sex life" may improve their body's immune system. Those who engage in sex once or twice a week have been linked with higher levels of an antibody called immunoglobulin A or IgA, which can protect the body from getting colds and other infections. "Scientists at Wilkes University in Wilkes-Barre, Pa., took samples of saliva, which contain IgA, from 112 college students who reported the frequency of sex they had." The college students in the "frequent" group had higher levels of IgA than those in the other three groups consisting of people who were celibate, had sex less than once a week, or had it "very often" (three or more times weekly).
Thirty minutes of sex burns 85 calories or more. While not seemingly a significant amount, 42 half-hour sessions will burn 3,570 calories, which is sufficient enough to lose a pound. A person can drop the pound in 21 hour-long sessions. "Sex is a great mode of exercise," notes a Los Angeles sexologist and president of the American Association of Sexuality Educators and Therapists. "It takes work, from both a physical and psychological perspective, to do it well."
"Boosting self-esteem was one of 237 reasons people have sex, collected by University of Texas researchers and published in the Archives of Sexual Behavior." In response to the findings, one researcher adds that those who already have self-esteem say they sometimes have sex to "feel even better". People have sex to feel good about themselves.
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. Researchers from the University of Pittsburgh and the University of North Carolina at Chapel Hill "evaluated 59 premenopausal women before and after warm contact with their husbands and partners ending with hugs. They found that the more contact, the higher the oxytocin levels". Oxytocin allows people to feel the urge to nurture and to bond. Generosity has also been credited and linked to a higher level of oxytocin. In addition, as the hormone oxytocin surges, endorphins increase, and pain declines. "In a study published in the Bulletin of Experimental Biology and Medicine, 48 volunteers who inhaled oxytocin vapor and then had their fingers pricked lowered their pain threshold by more than half." The oxytocin released during orgasm also promotes sleep, according to research.
Men who have frequent ejaculations, especially men in their 20s, may reduce their risks of prostate cancer later in life. Australian researchers reported in the British Journal of Urology International that they followed men diagnosed with prostate cancer and those without. They found no association of prostate cancer with the number of sexual partners as the men reached their 30s, 40s, and 50s, but men who had five or more ejaculations weekly while in their 20s reduced their risk of getting prostate cancer later by a third. Another study, reported in the Journal of the American Medical Association, "found that frequent ejaculations, 21 or more a month, were linked to lower prostate cancer risk in older men, as well, compared with less frequent ejaculations of four to seven monthly".
During sex, pelvic floor muscle exercises known as Kegels offer benefits for women. More sexual pleasure is expected to result, strengthening of the area, and help to minimize the risk of incontinence later in life.
In contrast to its benefits, sexual behavior can be a disease vector. Safe sex is a relevant harm reduction philosophy.
Social and other behaviors
In both 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.
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 apparently to 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.
Alex Comfort and others posit three potential advantages of intercourse in humans, which are not mutually exclusive: reproductive, relational, and recreational. 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).
Sexual acts, other than as a means of reproduction, are varied: Oral sex consists of all the sexual activities that involve the use of the mouth, tongue, and possibly throat to stimulate genitalia. It is sometimes performed to the exclusion of all other forms of sexual activity. Oral sex may include the ingestion or absorption of semen or vaginal fluids. Other non-penetrative sex acts are also common.
While there are many sexual acts involving the anus, anal cavity, sphincter valve and/or rectum, the specific meaning of anal sex is the insertion of a man's penis into another person's rectum.
Sexual ethics and legality
Unlike some other sexual activities, vaginal intercourse has rarely been made taboo on religious grounds or by government authorities, as procreation is inherently essential to the continuation to the species or of any particular genetic line, which is considered to be a positive factor, and indeed, enables most societies to continue in the first place. Many of the cultures that had prohibited sexual intercourse entirely no longer exist; an exception is the Shakers, a group that reached a size of about 6,000 full members in 1840, but as of 2006 had only four members left. There are, however, many communities within cultures that prohibit their members to engage in any form of sex, especially members of religious orders and the priesthood in the Roman Catholic Church and Buddhist monks. Within some ideologies, coitus has been considered the only "acceptable" sexual activity. Relatively strict designations of "appropriate" and "inappropriate" sexual intercourse have been in human culture for hundreds of years. These legal or cultural restrictions may include:
- Sex among partners who are not married (this is sometimes referred to as fornication)
- Sex between a married person and someone to whom they are not married (called adultery or extramarital sex).
- Commercial sex (also called prostitution).
- Sex between partners of the same sex (also called homosexuality).
- Sex between a living human and a human corpse (also called necrophilia).
- Sex between close relatives (also called incest).
- Adults having sex with children (depending on the country and its laws, also called child sexual abuse)
- Humans having sex with non-human animals (also called bestiality).
- Sex between members of different tribes, ethnic groups, or races, as in South Africa or the United States during periods of racial segregation (also called miscegenation).
- Sexual intercourse during a woman's menstrual period, as in Islam and Judaism.
Often a community adapts its legal definitions during case laws for settling disputes. For example, in 2003 the New Hampshire Supreme Court ruled that same-sex relations do not constitute sexual intercourse, based on a 1961 definition from Webster's Third New International Dictionary, in Blanchflower v. Blanchflower, and thereby an accused spouse in a divorce case was found not guilty of adultery based on this technicality.
Most countries have age of consent laws specifying the minimum legal age for engaging in sexual intercourse. Sexual intercourse with a person against their will, or without their informed legal consent, is referred to as rape, and is considered a serious crime in many cultures around the world, including those found in Europe, northern and eastern Asia, and the Americas. Sex, regardless of consent, with a person under the age of consent is often considered to be sexual assault or statutory rape. The age of consent varies from country to country and often by state or region; commonly, the age of consent is set anywhere between twelve and eighteen years of age, with sixteen years being the most common age the law sets. Sometimes, the age of consent is lowered for people near the same age wishing to participate in intercourse. For example, in Canada, the minimum age of consent for all couples is 14. However, the age of consent can go below 14 on the condition that the couple still are not 2 years of age apart. Religions may also set differing ages for consent, with Islam setting the age at puberty, which can vary from around 10 to 14. There are exceptions in the case of anal sex or people in a position of trust/authority.
Views on sexual intercourse, as of sexual activity in general, vary widely between religions, as well as between different sects of the same religion, and even between different members of the same sect.
The new and sudden substitution of the more ideal but extremely individualistic love motive in marriage for the older and long-established property motive, has unavoidably caused the marriage institution to become temporarily unstable. Man's marriage motives have always far transcended actual marriage morals, and in the modern era, the ideal of marriage has suddenly far outrun the self-centered and but partially controlled sex impulses of the races. The presence of large numbers of unmarried persons in any society indicates the temporary breakdown or the transition of the mores.
The ancients seem to have regarded marriage just about as seriously as some present-day people do. And it does not appear that many of the hasty and unsuccessful marriages of modern times are much of an improvement over the ancient practices of qualifying young men and women for mating. The great inconsistency of modern society is to exalt love and to idealize marriage while disapproving of the fullest examination of both.
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