I'd be for the legalizing of polygamy for the same reason I'm for same sex marriage. If you don't like those things, then you don't have to do them. If you're a man and want to marry another man? Knock yourself out. If you're a a man and can convince several different women to marry you? Personally, i think you're nuts, but whatever floats your boat. If you're a man who wants multiple gay marriages? Fuck it, who cares.

Which isn't to say I wouldn't think polygamists are weirdo's, but their weirdom has literally zero impact on my life.

And no offense, man, but "If man to man marriage is permissible, then next it will be man to child, or even man to animal." is just about the worst argument I've ever heard. Children and animals can't given consent, they don't make informed decisions; Same sex marriage is not some slippery slope to bestiality and child brides, claiming otherwise is just ridiculous.