The whole religious argument is really invalid unless these crazy religious people really want us to make the laws based solely on the bible, which they don't. They just want to judge people.
If they really wanted us to follow the bible, we'd be punishing people for having sex while a woman is menstruating not to mention that all women need to go away for 7 days every time they get their period, we should be giving sacrifices of meat to God, we should be stoning people for procreating outside of their race, tattoos are right out, shaving isn't a good idea either, mentally challenged people shouldn't be allowed to worship, and possibly worst of all WE WON'T BE ALLOWED TO EAT BACON!!!
What's really scary is how many people have been brainwashed to believe that the USA was founded on christian beliefs..
It's actually partially true. The people who came here from Europe did so to escape religious persecution so they could be free to practice their religion. The puritans were actually pretty intolerant of other religions, but that's not the way they wrote the constitution, fortunately.
Anyway, politicians love that this is such a heated moral issue so they don't actually have to admit the real reasons why gay marriage has not become allowable most places. It really is just a tolerance and civil rights issue. As dbzeag mentioned, it all comes down to money. We simply can't afford it currently. Our healthcare system will be effected further, small business would not be able to afford benefits, blah blah blah. I think it sucks. I think we should find a way to make room for this in our budget. I don't have all the answers though.
Gay marriage does put an interesting idea into the debate. If we legalize it, it could put the government in the awkward situation of having to force it's morals on religious organizations. For example, what if a gay couple wanted to get married at a place owned by Christians who do not morally agree with the couple. Do those Christians have the right to deny the gay couple access to their facility? Should the government force the gay couple on those Christians? Is that a violation of the separations of church and state? I dunno.