All right, sure.
It really depends on if the Gov't is in the marriage business. Because if marriage is CIVIL, i.e. gov't sanctioned, there is no reason to discriminate against two adults entering into a legal contract, sharing rights and taxes and stuff. Shouldn't matter if it's men or women or whatever, as long as they are legal adults of sound mind.
Most people that oppose gay marriage almost exclusively due to RELIGIOUS reasons. Fine for you, but your religion isn't my religion, so why should your rules dictate how I live?
That's the big issue. I marriages were ONLY from religious institutions, then fine, they can do how they do. If a marriage is a civil legal contract, then there isn't any particular reason to discriminate.