Maybe it begins with them. That is, I think, how it went with the War of the Triple Alliance. I forget whether it was Uruguay or Paraguay, but the first thing that happened was that 2/3 the male citizens got killed, and then polygyny became legal for a brief time.
Anyway, as with most of these discussions, it's between difficult to impossible to tell whether the arguments are real or alibis.
Mononormativity, heteronormativity, and all that "normal" stuff produces heinous amounts of suffering, but nobody invested in the assumptions of goodness of those things is ever going to criticize them. I could as easily say "if people are naturally monogamous and heterosexual and it works for them, then more power to them, but it just ends in things like Lorena Babbit's cutting off her husband's penis or women stabbing their boyfriends 32 times for playing a video game."
That would be unfair of me but no less unfair than picking out bits from polygamous societies and collecting them together in a folder labeled "What's Wrong with Polygamy." It might even be a bit more fair, as many of the so-called problems with polygamy can be solved by having about equal amounts of polygyny and polyandry, whereas nobody has ever figured out how to make monogamy always peaceful. (Nor is it obvious that people want it to be peaceful; go to a karaoke bar and look at how gleefully they sing those Carrie Underwood songs.)