There was a lot of push back by conservative intellectuals that argued that after homosexuality became legal polygamy would be next. Add to that America’s history of polygamy through Mormons and the fact economically two adult households can’t keep up in inflation and I think polygamy will become more openly practiced in a few years. The issue is if you thought gay marriage was tough to pass legally, imagine trying to make a polygamist marriage legal for the purposes of benefits, rights, and divorce.
So with that said I don’t know if polygamy could ever be legal. The fabric of our society is based on two person relationships when it comes to property, benefits, wills, and taxes. Adding a third, fourth, or fifth, etc person breaks those things. Gay marriage was avoided due to religious beliefs and/or discrimination. Polygamy is a system hack…
It may become more widespread, but the real ability to save and split bills would be if it became legal.