"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?