Let me try to be brief. The only part of a marriage that the government should be involved in is the contract itself. Who the contract includes shouldn't matter, as long as the individuals are of legal age and sound mind.
Better?
Plus, there's the problem of religions that sanction gay marriage. It's unconstitutional (IMO, IANAL) for the government to sanction marriage type A if that only works for some religions' marriages and excludes others'. (There are also religions like the Episcopal/Anglican church that are experiencing schisms over how deal with the raft of issues stemming from current LGBTQ cultural acceptance.
Now, you can use civil unions to give the same rights, but that ignores the societal impacts of allowing gay relationships to be called marriage.
Andrew Sullivan, a conservative (who is gay,
married (MA), and Conservative as in small gov't, fed up with the Republican Party, and who really hates the creeping Christianism and anti-intellectualism of the Republicans)
talks about how his experiences with letting LGBTQ's marry has led to an assimilation into the culture that wasn't previously happening.