It is true that as long as we have laws against bigamy no one can legally marry more than one person. Therefore, those who in seriousness express concern that allowing same-sex marriages opens the door for legalizing polygamous and polyandrous marriages are missing the point.
However, I'm not as sure about those who fear that legal marriage to a child, an animal or an inanimate object is next. They could have a legit issue here. That to me would possibly justify some kind of constitutional-level codification of what marriage is and who can legally marry. Wouldn't stop those who love getting one court to overturn the decision if another court or a legislative body but it'd help us begon to clarify things.