No, never. I'm always faithful even if the relation is going sour. Call it morale or principles or whatever, but if you have chosen for a relationship the least you can do is respect that relationship. You just can't go "shopping around" where you stick with your girlfriend because of an emotional connection, and spent your free time and your bed with another because she's great in bed.
If it's not working out you talk about it. And if certain things are that important to you (or her) that you cannot reach some sort of consensus, you brake up.