People get all testy when I start spouting off about how everyone is a secret scumbag that can be corrupted into cheating (yep, you too.) They think I'm accusing them of doing something or shitting on their morals, but I'm just being an Honest Abe. I know people who are 'that guy': dudes who are banging other guy's fiancees and wives with alarming frequency. For every girls night out, some broad meets a stranger, hooks up, gets shellacked, then calls her husband up with a 'good night honey. I love you' before cleaning up.
Men will cheat when their girl becomes a sweatpants (regular Rocky 2 style sweats, not even a flattering in the ass tracksuit) bore and women will cheat because they need constant attention. There is a simple solution to maintaining a good relationship: treat your woman like she's the hottest piece in the world and really believe it. If you can't or won't, your relationship sucks and one of you could and should do better.
Even in the strongest of relationships, nostalgia and the ex factor can lead to some catting around. People are inclined to make justifications that it won't increase their partner number or that old gag about ex sex is like riding a bike; but the truth of the matter is that nearly everyone is still carrying a strong flame for someone they practiced making babies with in the past.
Once again men and women have completely different takes on the matter. Guys get all sorts of flack for supposedly thinking with their dicks, but it's always 'the one that got away' that they long for. Men are super sentimental about their favorite ex. Over the course of time she has transformed from a chick you broke up with to the most perfect, virtuous, flawless goddess that ever lived. I can't believe how many dudes have created this fantasy woman in their mind, based loosely on a failed relationship. They amplify whatever good their was and completely forget about the negatives. That's the bitch he'll throw it all away for.
Women and logic don't mix, ever. The person from their past that they want to fill 'em up is the wrongest motherfucker they ever dealt with. The dude who treated them like shit, cheated on them, banged their friends and or family members is the one they want. Chicks love scandalous shit and being with this jackass again turns them on something fierce. Unlike men, they aren't trying to throw their current relationship away for an unstable misfit, they'll just bang him and keep it their little secret.