Historically polygamy has it’s foundations in a “family” where the single male/husband is utterly dominant (Alpha) while the females/wives are subservient, usually with the elder female/wife acting as the Beta. Or in some cases a co-Alpha.
IMHO the best political way to create opposition to polygamy is to emphasize the sexism, traditional gender roles and power differential in such relationships that reduce the younger/junior wives to slaves, sexual playthings and baby factories.
IOW turn the Lefts own language and rules against it.
But just think about women in general now. I don't think any woman would be a slave unless it's a cult. So maybe it has to be a cult for it to work.