The similarities between interracial marriage and gay marriage are numerous and powerful. In both cases those who wish to partake already have the "right to marry", just not the person that they want to.
I've said it before, and I've no doubt I'll say it again; giving the rights to two people of the same sex to marry in no way reduces the rights of anyone else in any way.
*Why* do some people want to ban it? Because it's "icky"? Because their religion disagrees with it? For the first, why is no one up in arms about banning licorice? I mean, that stuff's thousands of times offensive than any gay marriage; gay marriage doesn't trick you into thinking it's actually a normal flavoured jelly bean, only to leave you gasping for breath as you taste its awful licoricy flavour, and run gagging to spit out the disgusting substance into the bin...
And don't you think that if suddenly religious beliefs are made into law you might find yourselves not only when trying to marry the person that you love, but when you're trying to get an Ice Cream Crunchie (damn, they're good...) at the 7-11 on a Saturday (which is closed, of course, it being wrong to work on the Sabbath), or eat pork? What about when your girlfriend is arrested for not wearing clothing that completely covers her? And what if a neighbouring tribe rapes and pillages your town for Odin?
At what point is a religious belief so antiquated or so irrelevant that it should have no basis on law? Bishop Jones is not going to be forced to marry two men in the same way that Rabbi Goldenstein isn't going to have pork shoved down his mouth; but those who would like to have pork or marry whomever they love should be able to without a bunch of people trying to legislate something that doesn't concern them and has no effect on them.