Just what is lust, anyways?

Jacob Allen's comment Seraphim's and Jacob's comments about clothes and purity reminded me of this question. The issue of Matthew 5:27ff has always intrigued me. Is Christ saying what we think he's saying? I wonder. The stock evangelical answer is that it's wrong to look at a girl's legs for too long, or that young ladies ought to wear heavy, formless sacks which belie their forma. But look at the text.

"You've heard it said: Don't commit adultery. But I tell you that anyone watching [blep™n] a wife [gunaika] in order to desire [pros to epithum�sai] her, has committed adultery with her already in his heart."

Please note that gune may be read "wife." There is probably good reason to in this context. Note too that epithumeo is the same verb/root which the desire of our Lord to commune with the disciples is expressed in Luke 22:15 as "With desire I have desired [epithumiai epethum�sa] to eat the supper with you."

So from these passages, is it somehow impure or wrong to desire specifically a sexual, marital relationship with an unmarried, eligible woman? I don't think it is. It would probably be wrong, selfish, and silly to desire her in a depersonalized way. But to desire her, sexually and personally, because she's beautiful, female, and unspoken for--is that really at all inconsistent with what our Lord is saying in the Sermon on the Mount?