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.
I don't disagree that they don't go looking for the oppurtunity I'm saying that it's a moral failure to go for it when the oppurtunity arisies. Being morally flawless in the absense of temptation is no challanging feat. Your morals are what is supposed to make you say NO.