I'm not a literalist either--thank God. Too much contradiction for one poor brain to contain. Furthermore I look at the physical universe scientifically, which rules out a lot of Scripture as literal truth.
In answer to your question--Here, too, it seems to me that impulses of the heart ought not to be sins--just as you can't make vows about them, but only about your actions, you ought not to be held accountable for impulses you don't act on. And I think those laws in the Old Testament were meant to control people's behavior. Covetous behavior is disruptive to the community.
Of course, we have Christ saying that bit about someone who looks on a woman with lust has committed adultery with her in his heart. <flip, flip> Matthew 5:28. That's in the middle of some really tough talk from Christ {"If your right eye causes you to sin, tear it out and throw it away"), and I'm not qualified to expound on it. {Though is "adultery of the heart" a sin on the level of actual physical adultery? Or is it just dangerous because of what it might lead to?)
That enormous issue aside, whether or not impulses we don't act on are actually sinful, it certainly isn't healthy to live inside feelings like that for long. And inevitably there will be some bad consequence, even if it's just making ourselves ill. So we're taught to ask God (at least) for help when caught in a loop like that.
There--a semi-taught believer's take.