Mating in Private Places:

Wood from Eden:

"Among chimpanzees, the alpha male becomes outraged by seeing anyone else mating and usually responds violently when he discovers it. Sometimes he can create alliances with a few other males and allow them some mating opportunities too. But as a rule, alpha males hate seeing others mating. The alpha male’s own conspicuous copulation could be seen as a social marker. He is the alpha and he can do what others can not. Mating in public is a kind of status display, almost aimed at causing envy.

Not only males get upset by seeing others mating, but also children. Chimpanzee children in general do their best to disturb their mother's mating. They have good reasons to do so. Sharing the mother's resources with a sibling will make their own survival chances smaller. Postponing siblings is a good idea for infant chimpanzees. So they are doing their best to show their discontent when their mother takes up mating again after a few years caring for their child.3

Fundamentally, there was, and is, no solution to conflicts over mating. Other males dislike it, children dislike it. So humans did the second best thing: They pretended that it just didn't exist. If we just don't talk about the fact that others have sex, it at least arouses a little less resentment and conflicts."