How to argue against same sex marriage (part I)

In my contemporary moral problems class this semester we've been discussing same sex marriage.  We paid close attention to the Supreme Court oral arguments on Prop. 8 and DOMA last week.  We've been reading articles "pro" and "con"...and, honestly, it seems to me that all the best arguments are "pro".  I'm definitely in favor of legalizing gay marriage.  That said, lately I keep wondering: what