Its not the lustful thoughts that are the problem. Thoughts come and go, the problem is when we indulge them and nourish them. When we make things the focus of our entire mind and think of little else, that is what God considers wrong. Playing out senarios in our mind, sexual or not, for instance. I've read a book that talked about capturing our thoughts and locking them away. That is what we must do as christians. If a fleeting thought passes though our mind we are supposed to tell ourselves we aren't allowed to think that way, and then replace the thought with something else, say a bible verse.
As for the marriage part, I think the idea is that God has given the person to you and therefore they are yours. Lust is synonomous with Greed and coveting what isn't yours to have. Your wife or husband is yours just as you are theirs. That being said, even married people can get so focused on each other and the physical relationship that they forget God. Turn it into an idol of sorts. Single or married that is a bad thing because it is replacing God.
Great post, you make some good points. Nice work on defining lust too. I think it's important to specify what we mean when we use a word sometimes, because people often disagree only by misunderstanding what was trying to be expressed verses what was perceived by what was said. I guess in this thread when I have used the word "lust" it really wasn't the right word for what I meant. I was using it instead of .. I guess "sexual appetite" would be a better way to put it. I can see now that if that is the definition of lust - that it's about coveting something that's not yours, that it's ok to call it bad. That's a different story than what I've been babbling about though, I think... will have to give this some more thought, you've put a different, helpful spin on it