Well, if gay marriage and interracial marriage rulings are any example, polygamy might well become legal when about 40% of people approve of the practice. That sort of large minority isn’t happening right now, except maybe in Utah where they were forced to write a ban into their original constitution.
Realize for a moment, though, that polygamy laws come from the dominant culture. We may find it distateful, but polygamy is legal in the Muslim world because their religion allows it. It used to be legal in pre-state Utah beecause their religion allowed it. I think our problem with polygamy isn’t hippie triads living in San Francisco and abusing Vaseline, but cultures where women are bred to be property.