The only other reasons would be a backward, repressive view of sex (which usually goes hand in hand with radical religious views) or they like to legislate their view of morality, and which point they are totalitarian and run counter to the concepts of liberty and freedom this country is founded upon and should move to someplace more to their political liking... like Iran.
Incorrect. You are blocking out vast swathes of middle to upper class people, many of whom are not religious at all. The sort of folks who live more or less the good life, raise 2.5 kids, and send them off to college.
The unmentioned reason is understanding that regulation of our actions is what makes us civilized, builds homes and corporations, and establishes a solid foundation for future generations. It doesn't have to do with religion as much as not wanting to be like a caveman.
It is such evolved thinking that makes someone trade McDonald's for something a little more healthy, or take pride in appearance in order to be just a little more respectable; when a person thinks about saying something rash, but decides to speak courteously instead. It is the human ability and desire to graduate beyond born animal instincts and embrace a higher order.
Most of America believes in fidelity; it is the glue that holds together all organized society. It has nothing to do with faith or extremism.