What about the people who argue homosexuality itself (which would include gay marriage) is wrong because it isn't natural? Technically they have a point because if it were natural, two men or two women would be able to produce offspring.
Another interesting question is this: If evolution is true (jury's still out on that one, but that's another topic) then according to that theory homosexuality should have gone the way of the dodo a long time ago, as homosexuals cannot reproduce naturally and therefore cannot pass on their genetic material to another generation, so why are there still homosexuals at all?
Yeah, these are devil's advocates questions - MikeT23 is doing it, so I thought I'd get in on it.