This I propose is from a scientific perspective anomalous. Logically recreational sex should be homosexual like other recreational fun activities, perhaps even more so. It cannot because sex is done for reproduction since contraception is an important concern amongst those having straight sex. It might be argued that sex has nonreproductive functions as found in other primates. It is known, for instance, that in the pygmy chimpanzee, sex is used for cooling social tensions both within a group and homosexually when strange groups encounter each other.
Females engage in mutual genital rubbing.
Not only does such sex reduce social tensions but helps females to bond together. This empowers them against males by helping them to join into female alliances against them. Male sex has been seen with a social bonding in all male groups of rhesus monkey and gorilla suggesting the nonreproductive use of sex is widespread in higher primates. It could be argued that heterosexual sex has a similar function linked to the formation of parental bonds. While this is a possibility, it leaves unanswered why recreational sex has among humans become linked exclusively to the formation of these bonds.