Help me understand...

I've been a Reddit creep for a few months now (since the mass Digg exodus, heh), and I created an account today specifically to ask this.

This past election was viewed as a general loss for Gay Rights Activists. And that got me thinking.

What exactly is the logical, sane argument AGAINST gay marriage and rights? As far as I know, most people cite the Bible as stating that a marriage is a union between a man and a woman. But that argument can't actually fly in the government thanks to Separation of Church and State... right? And if so, what is the argument that they use?