SCIENCE: Why Societies Sanction Female Promiscuity:

Results of both studies were consistent with the theory that opposition to promiscuity arises in circumstances where paternity certainty is particularly important and suggest that such opposition will more likely emerge in environments in which women are more dependent economically on a male mate. . . .

In other words, the more dependent on men women are, the higher the reproductive stakes: If you get pregnant and can’t make a strong case that a given man was the father, you (and your child) could be in serious trouble.

This is a pretty old-school way of looking at things, of course, but that’s the point: Not all aspects of human culture are caught up to modern life. As the researchers write, these “beliefs may persist due to cultural evolutionary adaptive lag … that is, because the environment has changed faster than the moral system.” Today, thanks to modern contraception, when we want to separate sex and reproduction, we can do so with a very high success rate. Many of our older and more conservative ideas of morality may have developed before this was the case.