The ongoing trial of the Fundamentalist Church of Jesus Christ of Latter Day Saints, a breakaway Mormon sect that has been practicing polygamy for decades, shows what happens after polygamy gets established. The patriarchs of these extended clans raise girls for the purpose of giving them as teenage brides to other polygamist patriarchs. The girls are told it is their religious obligation to marry whoever their father picks, and be a dutiful wife and mother to him. Period.
Proponents of polygamy, like the ACLU, are imagining adults who are free to choose whatever domestic arrangement they like. First generation polygamy might even work that way. But after that, girls become a trading asset, passed from one polygamous clan to another. Even the young woman who testified in the trial, who was told by her father when she was 16 that he was marrying her off to "some Barlow boy" (he couldn't remember the name) the next day, was reluctant to testify. When asked to explain why to the mystified jurors, she said, "We were taught that we would go to hell" for speaking out. "After I got away from that religion, I still felt like I would be damned if I 'spoke out' because that's just how I was raised."
If ever there should be a feminist issue, fighting polygamy should be at the top of the list.