"Morally upstanding people are the do-gooders of society, right? Actually, a new study finds that a sense of moral superiority can lead to unethical acts, such as cheating. In fact, some of the best do-gooders can become the worst cheats."
http://www.livescience.com/strangenews/071114-cheating-basics.html
Most of the article refers to cheating as on exams, but I suspect that cheating in relationships (not that anyone here does that) would be similar.