I'm as atheist as you can get and I'm very much monogamous.
It's not some superstition that drives me to be only with my wife. To me it's a matter of commitment. When I accepted my wife, and she accepted me, I committed myself, mind and body, to her. I live my life, for her, and she lives hers for me. I don't believe I can truly feel or be committed to her if I lust after and engage other women.
She is the focus of all my energy. I work every day to improve her life as much as my own. When I contemplate an action, her benefit outweighs my own. I strive to make sure that in all things she can always know, without question, that she can trust me.
I will never bring home an STD or a child born of another woman. I can trust that when we do our thing, I won't be catching anything we didn't already have. What she does for me, and gives me, is for me alone. Just as what I do for her, and give to her, is for her alone.
I wouldn't change it for anything, and I truly feel sorry for people who don't share this level of commitment within their relationship.