I don't agree, I think it's plain simple moral character.
I don't think so. Most of the people that I know who have fallen to temptation clearly didn't set out to.
The more opportunity someone has for extramartial sex, the more likely they will have extramarital sex.
Morality has something to do with it, but not nearly as much as you think. Perfectly moral people fall to sexual temptation. Like I said, when they make that snap decision to have uncomplicated sex with that very attractive new person in their life, it really seems like a good idea at the time.