Sometimes I think it is out of boredom. Even if I have a friend who's wife is not very attractive, eventually I'll have fantasies about her. With one, I have had major fantasies and wish that we could have worked out a couples thing, but my wife is not in to that. I do not know if I would have an affair or not if the opportunity arose. It is just weird thinking that my friends might be dreaming of taking my wife in the backside, cumming on her face, etc., which I do with theirs.