Some argue that legalizing gay marriage means the government has to legalize polygamous marriages. Not necessarily so.
Denying gays the right to marry whom they love is discrimination based on an immutable personal characteristic, same-sex attraction, similar to forbidding people of different races from marrying. The Supreme Court ruled that miscegenation laws are an unconstitutional infringement on and individual's liberty. It should rule similarly for bans on same-sex marriages.
However, multipartner marriages, of whatever mix of sexes, seem to be based more on a lifestyle choice that can be regulated rather than an immutable characteristic that one shouldn't use as a basis for discrimination, such as race, ethic origin, sex or sexual orientation.