I'm not sure your conclusion follows from your data. Sequential monogamy isn't polyamory. There seems to be strong evidence that, given autonomy, most people aren't monogamous for 20+ years, but it doesn't follow that they'd stay with their first partner if there wasn't a social stigma against leaving them even if they could have sexual relations outside of that relationship.

My hypothesis is that most people are generally monogamous, but not to the same person indefinitely, and that transition is either marked by divorce/breakup if socially monogamous, or an odd transition period where they haven't really broken up with their last committed partner but have functionally done so if they're polyamorous.

I've certainly seen both ends of this specific spectrum work for those involved, but it seems about as general as anything else about human sexuality.

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.

There is multiple forms/degrees of monogamy.

Some animals have lifelong monogamy where they have one fixed breeding partner (many birds). Some animals have couples cooperate and raise offspring for a breeding season and then look for another partner in the next season, others mate for multiple seasons.

https://web.stanford.edu/group/stanfordbirds/text/essays/Mon...

Only about a quarter of divorce is related to infidelity.

Cheating on someone or even being an advantageous position where you can have more than one lover with your partner(s) having no recourse isn’t the definition of polyamory these folks are working with.