I think that Teigyr is mostly spot-on... I imagine getting to the root of "why" is something that a counselor would do. However, even if you get good counseling and you "fix" every aspect of your relationship and have a wonderful & happy home, a cheater will always be tempted to cheat... it's almost like a type of addiction. (I'm assuming that even if he's never actually cheated that posting something on a swingers site probably means he'd be willing to)

Oh yeah- and for the most part, swinger sites aren't populated by hookers and strippers... they're generally "normal" people that don't follow "normal" sexual rules. (not that it justifies what this guy is doing or anything)