I do believe that’s the case. It wasn’t how people lived prior to 10,000 years ago. This doesn’t mean that people today can’t choose it, but it’s entirely cultural — and some current cultures are more monogamous than others. The Bari of lowland South America are monogamous, except for when the woman is trying to get pregnant. Then she has sex with a couple of men.

Monogamy+is the type of polyamory that is the fastest growing in the US, I think. At least it’s the one I hear the most about.