It's completely unfair from any angle to judge normal relationships, especially modern ones, by these metrics.
Tho, it's not entirely clear to me how you come to the conclusion that a renaissance aristocrat wasn't just as much of a happy camper like the hypothetical, faithfully married commoner. Sure, there's lots of dysfunction to behold there, but I'm not sure that being raised by many different people and engaging in sex with more than one partner are signs of dysfunction on their own merit, aristocrats and for that matter modern ruling classes just happen to navigate and maintain larger social networks, if they're good at what they're born into - these kinds of fluid relationships might very well even correlate with IQ on some level.
Do these loose, far spanning intimacy styles arise from the power games they play? Sure, but then, they play those power games in the first place and even commoners, like Casanova, who rose through the ranks played them - successfully, another happy camper. This is surely lots of thunk food ... Queer theorists should def. stop dunking on faithful marriages, as if faithful, endearing elderly couples, who just happen to love each other are some menace, hell bent on ruining their fun.
I agree that probably a lot more people are bisexual than would identify as bisexual, if bisexual people are the middle of a spectrum more people would be there, but have you literally ever seen polygamy work out? I haven't.
I guess people, who do it well, don't brag about it, because of the stigma. And people rarely talk about things that just work for them, when they run the risk of social suicide. Technically, well behaved johns going to their prostitutes for years on end behind their wife's back are also doing polygamy. And that's a lot of people, who tell "cindy" all about their daughter's new school and wife's cancer treatment in between blowjobs. It's just a really shitty social "format" in which this happens. Here it's also noteworthy that edo japan had brothels AND geisha houses, in a geisha house, geisha's were well respected and had stable relationships with their sponsors, whereas in the brothels you basically had full on, all out sex slavery, it was possible to be sentenced to protitution as legal punishment and they had so many customers, they often died in their 20s of veneral diseases - i wouldn't call that kind of abusive, kinda anonymous (since nobody obviously wants to remember faces and partners change everytime) sex polygamy, but the john going to the same "cindy" for years is more like a geisha patron, whereas the john picking up streetwalkers is more or less a legal serial rapist, imho. The marker for functional polygamy is the emotional and intellectual ability to arrange and maintain actual realtionships with multiple lovers - on that sidenote: many archetypal gays also don't qualify - grindr hookups in public parks are something I'd also peg as dysfunctional, it's all fire an forget. Marriage should def. be shilled as less restrictive, overall people should be allowed to form social formats, which are a good fit for their personality style, so all the mentioned abusive stuff someday ceases to happen, which is still a long road ahead. But maintaining more than one relationships requires means and that's why poor peasants frequented the edo horror borthels and well to do nobles and merchants the geisha house.