Seeing as my ex left me for another woman (we were married 11 years), I think I can chime in here and say that being a freak in the bedroom does nothing to prevent someone from cheating, because affairs are not always about sex. I've talked to a lot of people through support groups who have cheated or have been cheated on, some of whom repaired the relationship and some of whom did not, and a good portion of the time people cheat because they are insecure in themselves and need an ego boost. It's not about their partner at all. People also cheat if emotional needs are not being met and/or if they are under a lot of stress.
Personally, I think monogamy is a choice that people make that is not altogether natural for the human species. I think that spending inordinate amounts of time alone with people of the opposite sex can lead to attachments forming, no matter how much you love your partner.