Answer: there should be none! Who cares what one person's sexual orientation or religion is? That's what America's about. Freedom. Those against it aren't going to take advantage of that freedom, so why bother making it illegal? This country has the freedom of religion, so why try to push some religion on the rest of us?
I know I will never even think of beniffiting from samesex marriage on a personal level. I'm straight. And, technically, I am a Christian, though I don't beleive in everything many Christians do. I've never had a problem with homosexuality. Just don't try to marry me, and I'll be fine. If it's not hurting anyone, and people are in fact benifiting from it, how could you possibly want to ban it? Even if religously you believe it to be wrong, let them find out on their own then.
Edited: Also, think of this, as Chibi_riki said. With more same sex marriages, there willbe more adoptions. With more adoptions, there will be less children in oprhanages.
Also, there is another thing. Many Christians are afraid of being outnumbered by Non-Christians. The problem is, many churches don't believe that someone can be gay or bisexual and still be Christian.