The only law I ever gave related to lustful thoughts was the law of coveting something that belongs to someone else. If you want to take something away from someone else and make it your own, be it his wife or his property, you are coveting. If you simply look at his wife and think of her lustfully, but you do not want to take her away from him, you have not coveted her and therefore have not broken any commandment I ever gave to anyone.