There’s a bit more to polygamy historically than presumption that men can’t stay faithful. It has more to do with economy and, indeed, ensuring the maintenance of someone who would otherwise be abandoned.
Well, I’ve also read that one of the likely practical reasons for polygamy, and specifically, polygyny, in Muslim countries at least in the Middle East, is that the region had so many wars back then, that men got killed in battle at a very high rate, so there was a very unequal female : male ratio, leaving many women without any means of support.
However, though certainly there’s still a lot of war going in that region, I doubt the male-female ratio is so bad these days that it justifies polygyny still being a practice. And it is not a justification I have heard from most Muslims themselves.
Although, unlike the FLDS, polygyny is not considered a requirement for Muslims, just an option, and comes with the caveat the husband must be able to financially support each wife equally, so only the richest people can afford more than one anyway. Most Muslims I know personally are of South Asia or Indonesian background and seem to see polygamy as an old fashioned outdated custom that’s just not relevant to their own lives. But on the other hand there was some self-selection going on, I assume they knew when they immigrated to the US that polygamy was illegal.
Anyway, I can easily imagine a Christian woman telling herself: ‘If I don’t accept his mistress, he will abandon me. If I invite her into the triangle, I can at least retain some control.’ Obviously works for a man too. Bisexuality (which is not a straightforward 0/1 thing) complicates matters a bit if both spouses are actually attracted to the stranger.
In any case, Christian marriage is not about fulfilling one’s fantasies etc.
Also, regarding polygamy as an alternative to abandoning the first wife, I can think of a really disturbing book I flipped through once in an airport that had a little bookstore. It was about a little girl growing up with her parents, siblings, and a mentally disabled woman who she was told was a distant relative of her mother’s - and that might even have been true, but it was not the whole story. She later found out that the woman was actually her father’s ex-wife, who was in a terrible accident that left her brain-damaged.
Now I’m not sure what the exact circumstances were here, I didn’t actually buy the book and I had to leave the bookstore to catch my flight. It seemed that the girl’s father was still married to the first wife at the time of the accident, and felt obligated to take care of her for the rest of her life in her own household But apparently, NOT obligated to actually stay faithful to her.
The girl noticed that the woman, while barely verbal, seemed very sad and unhappy all the time. Well, of course she’d be! I felt horrible for this poor woman, assuming she still had some memory of her prior life with her husband, to be forced to live with her ex-husband, new wife, and their kids, and be unable to do anything about her lot in life, due to her disability.
And since the little girl herself still saw her father as a great guy for not shipping her off to an institution, she was apparently forced to deal with everyone around her expecting her to be
grateful that her ex-husband was still taking care of her.
Now this wasn’t technically polygamy as the guy divorced the first wife before marrying the second one, and maybe it was still better for the first wife than being institutionalized, but I certainly wouldn’t consider it an ideal state of affairs to aspire to.
ETA: I realized I forgot to tell you the book was in the NON Fiction / Memoir section meaning a real person actually suffered in this situation.