I am fairly certain Hindu religion doesn't speak to homosexuality, yet the hindus I know are all against gay marriage.
While I believe in God, I have not been to church in 20 plus years - so, I am by no means religious. I don't believe gay marriage should be legal.
Lastly, someone I used to work with is an atheist. He isa republican, and is against gay marriage. I haven't seen him since he left he job, so I guess it's possible he's changed. But, based on conversations i've had with him, I doubt it.