I mean why would they if they only believe marriage to be between a man and a woman? I DON'T see it that way-as far as I'm concerned, marriage is a legal issue and as gay people already have civil partnership which offer same legal rights as marriage, I find the moaning about gay marriage irritating.
But that is besides the point: the fact is that some religions only believe that marriage is between a man and a woman and it is ridiculous to force them to change their minds without seriously messing that religion up.
People cannot expect religions to mould themselves to a nice politically-correct world. I accept this as an atheist. It's time the religious did, too.