I think there are a lot more "spectrums" of human quality than are typically aknowledged by society, including things like sexual orientation, gender identity, introvert/extrovert behavior... (I actually had this thought while discussing the autism spectrum with my sister, which several of my relatives are noticeably and demonsterably on.) I wouldn't doubt that an inclination towards monogamy or polyamory is also one of those.

And while I'd agree that cultural pressure plays a role pushing people towards monogamy, the impression that I've gotten is that, even in happily-accepting, supportive social groups, polyamory is hard. Monogamous relationships are hard, just because they involve two people with seperate brains who think in different ways. More people, more complications.

(This is also biased, of course, by the "case studies" of my friends. Of four people I know who've been involved in poly relationships, only one has gone smoothly and without horrible drama.)