My thought is that enforced monogamy came with the invention of agriculture, and wasn’t really a thing in Hunter-gatherer societies, but I could be completely wrong about that.

If true though, the fact that most of us don’t work in agriculture any more could mean that we are in the midst of a sea change in relationships. I’m too old to say I’ll be around to see how it ends up, but some of the youngest here might be around to see it.

The usual statistic that we hear from people who study genes is that about half of the men are fathers to women. It's a significantly different statistic from OKCupid / Tinder matches, which is closer to 80-20, and the fact that less young men are having sex than before suggests that it translates to real life.