I thought this would be the default case too, but it's surprisingly nuanced.

One red flag is anyone wanting to open a monogamous relationship that isn't more-or-less rock solid. Polyamory isn't something to try if you're uncertain about your current monogamous partner.

The ideal case is for someone to go into the dating pool as openly polyamorous. But this will take a couple more generations, I think. Most people don't have the opportunity, because the idea that we need to choose a single partner has been hammered into us from the moment we were born. So I'm sympathetic to people who want to put in the work of exploring whether polyamory is possible within the bounds of an existing relationship.

The most important thing is communication. There will be misunderstandings, but at no point should someone feel like something is being hidden from the other. That's different than respecting each person's individual relationships – one is underhanded, the other is a social contract.

This isn't directly related to your point, but https://www.unicorns-r-us.com/ was an interesting read.

It seems inevitable though. You can’t simultaneously be invested in a relationship and indifferent to its nature. This article points out a ton of issues and largely suggests just awkwardly independently dating other people as safer. Like “if you agree that everyone’s free then everyone’s free”. But… no that sounds dumb to me. I would not bet on such myriad bonds holding up in times of difficulty. Surely it can happen. But I just don’t buy it as a sensible strategy

It's like that scene from Arrested Development.

https://youtu.be/7NnW7AA9STg?si=r32PdL56dR7NVQL4&t=39

Monogamous divorce rates are ~50% first marriage, ~60-70% second marriages, ~70-74% third marriages. People are already polyamorous, just masking it due to social convention and signaling.

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.