At least for humans, this most basic of acts is anything but basic. As the pioneering sex researcher Alfred Kinsey put it, the only universal in human sexuality is variability itself. Within the universe of intimacy and pleasure that sex affords, however, there's a lot of room for error.
Our bodies evolved over eons, slowly calibrating to the African savanna on which 98 percent of our ancestors lived and died. So, too, did our brains. Evolutionary psychology postulates that the mind is shaped by pressure to survive and reproduce.