You can't always control the thoughts that come into your mind. What you do with those thoughts is a completely different story. If you are married and you see a girl with a nice butt and the thought "dat ass" pops into your mind, then there is nothing wrong with that. Now if you start fantasizing about how you want to screw her and can't stop thinking about it, that's another thing. It wouldn't necessarily be cheating, but at that point I would say it would be wrong. Now if the thought "I'd like to screw her" just popped into your mind and you were like "but that's wrong" and you force your mind to think elsewhere, then that would be the right thing to do. Unless something physical happens it's not cheating or anything. But I think if you keep those lingering thoughts there and dwell on them then it can be potentially damaging and wrong.