Why do liberals hate polygamy? If gay marriage is acceptable because of 'two consenting adults' then why not the same for a whole bunch of adults consenting to being married to one person?
I don't think it's a liberal thing to hate polygamy, pretty sure almost everyone in the U.S. is pretty biased against it. Many liberals probably take anti-polygamy stances because people will be that much more opposed to gay marriage if they think that polygamy will follow in 30 years.
Of course, if you want to define marriage as a bond between any 2 consenting adults, then there's really no reason to limit it to 2, as long as they're all consenting adults. Though in practice, don't lots of Mormons end up marrying minors?