Sexual desire is a strange beast. It can make otherwise level-headed and respectful people NOT be level-headed and respectful. And so it goes for women (and men) actively straying outside the relationship.
It's easy to condemn or be condemned (depending on what one has written here), though I have to look at the underlying situation if someone is 'cheating'. Any time I have strayed, the relationship I was in was already in deep decay. Trying to be honest with oneself and getting the guts to end things can be much harder than dealing with the act of straying.
I am with a very loving and caring fella right now, and intend to be with him for a very long time. We both feel that we don't need outside partners to fulfill our desires, and I feel no need to look beyond what is in my own bedroom for sexual fulfillment.
I know that the heart can imprison the body, but we should also be mindful of not being a prisoner of our own hearts. I guess constant mediation & reassurance with your other half is the only way to achieve a sustainable and monogomous relationship. Sexual temptation is everywhere!