A lot of times when armchair evolutionary psychologist types talk about sex, they only really seem to consider procreation, but I think at least in humans sex is also supposed to be important part of how we bond together. Eating together, sleeping together, jerking off together...these all help create a greater feeling of closeness with others, and in turn those social bonds make us happier and better adjusted.
@hiphoppers God, may I reincarnate as a bonobo.