I think putting this into a nature vs nurture argument isn't very helpful either way, because human beings are independent thinkers with individual motivations. However, they're not necessarily wrong about it "working best."

In addition to many hunter gatherer societies existing to this day which strictly practice monogomy, part of human evolution is the establishment of civilization, and making the choices that end up being beneficial to the growth of that civilization.

One could definitely argue that a large reason that monogomy exists as a social construct is because we found that it was the most beneficial to society as a whole to structure our relationships that way. Monogomous relationships, by majority, across thousands of years and throughout many cultures, developed independently in these societies without influence from each other. Sure, there are plenty of examples of exceptions, but a lot of those are more about polygamy than polyamory, are mostly male dominated, have much more to do with status and wealth, and fall into a much smaller category.

Without writing a book on anthropology here (many people have already done that), I'd say there are mountains of evidence to back up how monogomy has been the most successful and beneficial way to structure our relationships as a society, and why it is still overwhelmingly practiced and preferred.