What Is Lust?

I have a remarkable tendency to produce profound statements in obscure Facebook chat conversations. I need to make a habit of copying these down more often. Here's something from a while back, in response to a question from another guy, about what I though lust was.

Well, theoretically, it comes down to "desiring someone sexually." The question of course, is, what does THAT entail? Is it visualizing them naked or in a sex act? Is it in not doing that, but getting turned on when you look at them? Is it in imagining them in a relationship with you, ultimately leading to marriage, no matter how unsexual the fantasy might be in your mind at that moment? Is it solely limited to masturbating with the thought of them in your mind? Is it so broad that it encompasses even simply wanting to be with them in a relationship though you are not?

Clearly you can see that I think about this all the time, as well. I tend toward thinking, if you gotta ask, God is probably stricter about it than He is lenient. It's a purity issue. You can look at it from a 'fruit' perspective, as well -- does dwelling on someone in your mind all the time result in good things for you, or does it distract you from academic pursuits and lead to emotional havoc? If the product of thinking about someone in a certain way does so, then that could mean that what you were doing was not good--and if not sinful, at least not beneficial.

Verses referenced:
Matthew 5:27-28 -- ²⁷ “You have heard that it was said, ‘You shall not commit adultery.’ ²⁸ But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart. 
Matthew 7:18 -- A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit.
1 Corinthians 6:12 -- All things are lawful for me, but not everything is beneficial. All things are lawful for me, but I will not be mastered by any thing.

~ Rak Chazak