Sexual intercourse (synonymously coitus or copulation) is the act of inserting the erect penis of the male into the vagina of the female. Sexual intercourse and masturbation are the two most common human sexual activities, with sexual intercourse being the primary step in sexual reproduction. Sometimes the term "sexual intercourse" may refer to any sexual activities; see human sexual behavior for discussion of this broader sense.
Sexual intercourse is usually preceded by foreplay which leads to sexual arousal of the partners, resulting in erection of the penis and natural lubrication of the vagina. The erect penis is inserted into the vagina and one or both of the partners move back and forth to stimulate themselves and each other usually to the point of ejaculation and orgasm. A detailed description of the physiology of sexual arousal and orgasm can be found in the article Human sexual response cycle.
Sexual intercourse may be performed in a wide variety of positions, the most common ones being:
The woman lying on her back, the man on top and facing her ("missionary position")
The woman on hands and knees, the man kneeling behind her ("doggy style" or in Latin coitus more ferarum "sex in the manner of beasts")
The man on his back, the woman kneeling or squatting on top (slang "cowgirl")
Both partners lying on their sides, the man entering from behind ("spooning")
Coitus is the basic reproductive method of Homo sapiens as of all mammals. During ejaculation, which normally accompanies the male orgasm, a series of muscular contractions delivers semen containing male gametes known as sperm cells or spermatozoa into the vault of the vagina. The subsequent route of the sperm from the vault of the vagina, is through the cervix and into the uterus, and thence into the fallopian tubes. Sperm cells can survive up to nine days in the female body. 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.
Sexual intercourse should always be considered likely to result in pregnancy unless adequate contraceptive (birth control) measures are in force. Even then, pregnancy should be considered a possible outcome of the activity since few birth control measures are 100% effective. Coitus interruptus, or "withdrawal" of the penis from the vagina just before the man's orgasm, cannot be considered an effective method of contraception and is not recommended. Outercourse, in which there is sexual activity without insertion, can be performed without resulting in pregnancy provided that semen does not come in contact with the vulva.
Sexual intercourse, like other sexual activities that involve the possibility of transfer of body fluids, is also a means of propagating sexually transmitted diseases. Health care professionals suggest that condoms should be used to lessen the risk of contracting STDs, but they should by no means be considered an absolute safeguard. The best suggestion is to avoid sexual intercourse with anyone known to have a sexually transmissible disease, and, indeed, with anyone whose disease-negative status is in doubt.
Unlike some other sexual activities, sexual intercourse itself has rarely been made taboo on religious grounds or by government authorities. It is assumed that all of the cultures that prohibited sexual intercourse entirely no longer exist, such as the Shakers, an extinct sect of Christianity. Within some ideologies coitus has been considered the only "acceptable" sexual activity. Relatively strict designations of "appropriate" and "inappropriate" sexual intercourse have been almost universal in human societies. These have included prohibitions against specific positions, against intercourse among partners who are not married (this is called fornication) or are married, but not to each other (called adultery), against incest, and against intercourse during a woman's menstrual period.
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 called rape and is considered a serious crime in most cultures.
It is said that dolphins, bonobos, and humans are the only animals that have sex for pleasure. It is also said that only in humans and a species of spiders can sexual intercourse be forced.
