Sexual intercourse is the pairing of opposite-sex (or hermaphroditic internal fertilization animals) for copulation. The primary biological purpose of sexual intercourse is the creation of offspring.
Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization. The two individuals may be of opposite sexes or hermaphroditic, as is the case with, for example, snails.
In humans, unlike most animals, sex may or may not be related to reproduction. In most cases people have sex for pleasure; this behaviour is also seen in some animal species, for example chimpanzees are known to have sex when the female is not fertile, presumably for pleasure, which in turn strengthens social bonds.
Instinct of a male to have sex plays major role in sexual intercourse, in turn in reproduction 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 peoples' 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 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).

Main article: Oral sex