It's really a nonissue, though. If they can't come up with a reason that isn't religion, they lose automatically.
It actually kinda simplifies things.
"Because God said so" renders your argument automatically invalid.
At some point I delivered a speech to my fellow classmates on the subject. I acknowledged that religion plays a strong part in someone's beliefs and values, which is why separation of church and state is difficult at best. However, you cannot prohibit an action based purely on tradition and/or religion. You need a pragmatic reason. If it has a good reason to be done and happens to coincide with religion, fine. Otherwise, GTFO.
Also, the Bible doesn't just prohibit same-sex marriage:
Leviticus 20:13
And if a man lie with mankind, as with womankind, both of them have committed abomination: they shall surely be put to death; their blood shall be upon them.
Nice double-standard, people. Unless you're ready to start killing homosexuals, don't for a moment consider religion to be a valid argument. It just embarrasses yourself.
Over the past years I've had about enough of holier-than-thou zealots thinking they can step on the toes of other people. Not all religious people are like this, thankfully, but enough are for it to be a serious problem.