I don't support the old selling-daughters-to-older-guys form of religious/social polygamy. That has no place in a developed country. But an equal relationship between more than 2 people isn't anyone's business.
The fact that you expected this to be some kind of barn burner question to all of us belies the problem with government-instituted marriage: it's nothing but a way of enforcing narrow-mindedness about other people's personal lives.
Marriage didn't become a government institution in this country until after the Civil War. Why then? So they could ban interracial couples from marrying.
It's founded on bigotry, and that tradition continues, with each group that's excluded from their legal rights having to subject themselves to social intolerance before winning what should have always belonged to them in the first place: the right to conduct their personal lives without disadvantage or loss of rights to their family, simply because the mainstream can't handle the idea of people who aren't like them.