I don't know if this is completly accurate, but this is AskMen top 10 list.
10. Your lady doesn't put out
Every man has heard that the best way to get a woman to stop having sex with him is by marrying her. Apparently, long-term relationships seem to suck the sex drive out of many women, leaving men gasping for fulfillment. With the need for more sex, some men will start "working late" more often.
Or it could just be that the sex has become boring. She doesn't want to try new things in bed, or won't perform fellatio. Some men may cheat because they don't want their girlfriends or wives to perform certain sex acts that would ruin their "good girl" image, so they get a mistress to take care of it. Like Robert de Niro said in Analyze This ,