Why are some people attracted to the same sex? It's not unrelated to why people are attracted to anyone at all, and that broader topic is an ongoing one in scientific research.
Life as an organic replicator seems as good a shorthand one as any, though even if a particular organism is incapable of replicating (some defect in its biology) that wouldn't make it not alive, would it?
The notion that professing to lack belief in something presupposes you had to believe in the existence of that to begin with (and moreover, that the object had to be real also) falls completely on its face.
But for those who could suffer backlash as a non-believer, you could answer in a way that is technically correct, something like, "I don't think I attend the church you do," smile, and change the subject to something less controversial, like politics.