That belief system is actually only about 6–9 thousand years old, coming squarely out of patriarchal dominance hierarchies that arose about the time of agriculture. Our prehistoric ancestors believed in cooperation, sharing for the good of the group, cooperative breeding and alloparenting — a lot closer to modern poly folks than one might think.
“If sexual monogamy were “natural” and something that most people were innately inclined to, we would never have had or needed the social and sexual control of women that only arose 6–9 thousand years ago. But even beyond this, there are other ways that we know that sexual monogamy is a relatively new human expectation.
“A female who mates with several different males will have more genetically diverse offspring, boosting the chances that at least some of them will thrive.” Source This goes for human females as well as other non-human animals and it is confirmed in the anatomy of both human men and women.”
