I would argue that beyond genetics, it does in part have to do with your personal experiences. Everyone in the world sees different things, feels different things and being straight or gay is not 100% due to genetics, but in part personal experience. I would imagine any factor of life, be it personal experience, or genetics shapes who you are.
Background: I'm straight, non-religious, and have no hard-feelings towards straight or gay people. I just honestly believe that no single factor (though genetics is probably most influential) moves you into being straight or gay. I believe a culmination of all factors is the ultimate determination. Argue on.