Sexual behavior is required for reproduction in internally fertilizing species but poses significant social and physical risks. Females in many nonprimate species have evolved physical and behavioral mechanisms restricting sexual behavior to when females are fertile. The same hormones producing female fertility also control these mechanisms, assuring that sex only occurs when reproduction is possible. In contrast to nonprimate mammals, hormones do not regulate the capacity to engage in sex in female anthropoid primates, uncoupling fertility and the physical capacity to mate. Instead, in primates, sexual motivation has become the primary coordinator between sexual behavior and fertility. This dependence upon psychological mechanisms to coordinate physiology with behavior is possibly unique to primates, including humans, and allows a variety of nonphysiological influences, particularly social context, to regulate sexual behavior. The independence between hormonal state and sexual behavior allows sex to be used for social purposes. This complex regulation of primate sexuality develops during adolescence, where female monkeys show both hormonally influenced sexual motivation and socially modulated sexual behavior. We present findings from rhesus monkeys illustrating how social context and hormonal state interact to modulate adolescent and adult sexuality. It is argued that this flexibility in sexual behavior, combined with a tight regulation of sexual motivational systems by reproductive hormones, allows sexual behavior to be used for nonreproductive purposes while still assuring its occurrence during periods of female fertility. The evolutionary pressures that produced such flexibility in sexual behavior remain puzzling, but may reflect the importance of sexuality to primate social attraction and cohesion.With respect to sexuality, I have often speculated on it, and have always concluded that we are too ignorant to speculate: no physiologist can conjecture why the two elements go to form a new being, and more than that, why nature strives at uniting the two elements from two individuals. (Darwin, 1861, in Darwin, 1903 In the preceding quote, Darwin's reference to "sexuality" was in the sense of sexual reproduction. We are, however, similarly ignorant about the behavior that accompanies sexual reproduction, the now conventional use of the term sexuality. What are the causes for engaging in sexual behavior? Why does sexual behavior vary across development and across species? What brings two individuals together so that their gametes can combine? We have partial answers to some of these questions, while answers for others remain elusive. However much of what is understood reflects the reality that sex is expensive, dangerous, and time consuming.