Polygamy advocate Mark Henkel asks a powerful question: “If it’s all right for Heather to have two mommies, then why can’t she have two mommies and one daddy?” His challenge provides perspective on current demands that government endorse same-sex marriage. Why should society support the novelty of gay relationships ahead of polygamy, which was practiced nearly everywhere for thousands of years and would probably appeal to far more people than homosexuality?
The right answer to polygamists should be government neutrality: if private relationships involve consenting adults, then it makes no more sense to prosecute a male who claims he has two wives than to go after a guy who boasts of two girlfriends. But that doesn’t mean government should license polygamous relationships, any more than it should sponsor homosexual coupling.
In both cases, those who choose unconventional alternatives to one-man/one-woman marriage—still the best situation for child-rearing—shouldn’t be punished, but they shouldn’t be promoted either.
That argument is logical, but I'd be weary of bringing it up with your liberal friends lest you get your head chewed off.