I think the reason people assume atheists are more likely to be liberal (especially in regard to gay marriage) is because they have absolutely no reason not to be. Certain religions say being gay is "sinful," but why on earth would an atheist choose to hate on consenting adult people wanting to be married? What is the reason? (Seriously... atheists who are pro-treat-gay-people-like-trash - what is your reason? At least believing that a super-judgmental-ultimate-creator-and-destroyer being will hate our planet if gay people are treated equally is a reason that could legitimately terrify anyone who believes it.)
I can, however, see why anyone, religious or not, might be against alcohol (as it can be dangerous and kill people,) abortion (as it can be dangerous and kill people,) or promiscuous sex, (as it can be dangerous and kill people.)
No, seriously. I never knew being against alcohol was a conservative thing. I didn't know anyone was actively against it. I always saw it as more of a thing some people do and some don't. I never saw it as political. And abortion, while most who disagree with it are religious, I can totally understand someone disagreeing on a non-religious basis. I myself am against late-term abortion, and am extremely atheist. As for promiscuous sex? I'm not really sure what you mean by that...