Agree completely with radicalnan - people are wired differently, and some require more excitement in their lives than others. Excitement is part of the 'cheating' experience, whether it's via sex, shoplifting, or in a game of cards. In marriage, how would you ever really know if a spouse who has been found cheating and forgiven, subsequently remains faithful. S/he may genuinely realise that s/he has much to lose if repeating the behaviour and resist all urges, but I very much doubt whether this happens in the majority of cases - it may just be a question of being more careful not to be found out. Personally, I have worked with several men and women who I know to have been serial cheats, and yet managed to hold their marriages together. The fact that they behaved this way shows that they valued their marriages and the children coming from them, but that those marriages didn't give them the thrill they craved for. It's a human condition unfortunately.