It's not just serial monogamy, it's also infidelity, having an emotional relationship with someone without it turning into infidelity, etc. Agree the single data point alone wrt divorce rate does not lead us to my thesis, but in aggregate, it appears humans are not made for long term aggressive monogamy. That is what I meant by "People are already polyamorous"; they already want to or do love (or some other combination of relationships stronger than platonic friendship) more than one person at a time.

But I'm not sure even that follows. Most of the folks I know who got a divorce or cheated on their partner generally weren't in love with both people at the same time. Obviously it's hard to ever get an true answer on some of this, since it's hard to definitively say if you're in love with someone or not, but almost by definition, infidelity in monogamy isn't polyamory. I just had a friend who was in a polyamorous/open relationship go through a bitter divorce because of the way that a new partner was brought into the group and feelings of betrayal and infidelity that that caused.

Polyamory isn't some sort of magic flag that makes things not infidelity, and most outwardly monogamous people who are in relationships with multiple people aren't closeted polyamorous people, they're just selfish/abusive partners.