to answer the post between the time i started writing and now: If you ask me, polygamy can't possibly have anything to do with love, which is also inherent to family function. i didn't mention that overtly in this post, but love is one of those inherent family factors that holds it together where physical issues don't.
But who are you to say it isn't love? Ugh, I'm not trying to defend a polygamist, but I'm sure they'd argue with you on that one. So their value system is a bit different than most people's. If marriage is no longer defined anyway, you cannot deny their right.