Why are some people faithful and others unfaithful? Do men and women have different motivations? How does fidelity play into your view (e.g., RP, BP, etc) of male-female relationships?

I bring this up because TRP is obsessed with female infidelity. On the flip side, when I talk about counter-cultural views on marriage and divorce, I read a lot of "what if he cheats on you?!"

Yes, infidelity is a large problem for many couples. What we must remember, however, is how many people who choose not to cheat. My question is why? What separates the wheat from the chaff on this issue?