What I don't understand is why people that don't believe in God even support marriage, let alone abstinence. I've even run into Christians who think that an Atheist is someone who knows about God, knows that Jesus is the Son of God, but decides not to accept him as Lord and Savior. Or something like that. To such a person, the existence of God is an a priori belief so deep that they can't accept even the possibility that He might not exist. And if they do understand that others might not believe that God exists, the idea is so foreign that they don't understand the ramifications of that.
Atheists are quite varied.
That part's fine, but it doesn't say what they do believe in. So the question can't be answered for atheists collectively. Rather, it refers to a positive belief that the physical universe, the matter in it, and the forces which govern how that matter interacts are all that exist. The mind is an emergent property of brain tissue, but that's all it is. To a materialist life is nothing but a complex self-sustaining pattern, easily disrupted and gone when destroyed.