Yeah, I agree that no currently practiced religions allow gay marriage. Only dead religions (like polytheistic Greek and Roman), and even that wasn't about marriage but more about societal acceptability.
If you're going to grant all the same legal rights to a civil union that you are to a marriage, the difference is only semantical from a legal perspective. But the issue is not really about what to call it; it's about whether those legal rights should be recognized at all, regardless of what the relationship is called. I get the impression that most people who oppose gay marriage don't just have a problem with calling it "marriage," they have a problem with the LEGAL recognition of any rights attendant to this type of relationship.
Let me try to flesh out the contradiction I see a little more:
From a SOCIETAL perspective, most people are opposed to an uncle marrying a niece, but think it should be LEGAL if they are exercising their RELIGIOUS beliefs.
From a SOCIETAL perspective, most people (I guess) are opposed to a gay man marrying another gay man, and think it should be ILLEGAL even if those people are not violating their RELIGIOUS beliefs (i.e., they're athiests or whatever).
So most Christians think it's okay for some people to do something that violates their Christian beliefs if it comports with their particular (religious) beliefs. But most Christians think it's NOT okay for other people to do something that violates their Christian beliefs even if it comports with their particular (non-religious but moral) beliefs. I think it's contradictory because there is no NECESSARY reason to favor Jewish beliefs over Gay beliefs. Both practices are IMMORAL in the eyes of a typical Christian, but one gets deference while the other is condemned, even though both are horrible sins according to Christian law.
I'm not trying to rag on Christians in particular because I'm all about Jesus and . But I just don't see why religious beliefs factor in so much to the LEGAL discussion, and that there also might be a contradiction in the way Christians apply their religious beliefs to the law.