Dr. P. linked to an interesting article by a former student of his, written on the topic of understanding Matthew 5:28. He makes a case that the better translation is not "whoever looks on a woman with lust has already committed adultery with her in his heart", but "whoever looks on a woman to lust has already committed adultery with her in his heart". The point of this exegetical move is first of all its intended accuracy to what Jesus said, and second to remove the burden (if it can be legitimately removed) of what Jimmy Carter once called the "almost impossible standards" set there, that we can stumble into sin but not deliberately. The post also discusses whether being tempted is, in itself, a sin.
I am not here taking exception with the translation issue, but I do take issue with the exegesis and theology. I do not think we can say that, in "looking on someone to lust", that the lust is blameless so long as it was not in the original intention when we looked. No doubt that having the original intention of lust and that driving the looking -- as in pornography -- is a deliberate sin. But Scripture has never limited sins to deliberate sins. The post makes the point that being tempted cannot be inherently sinful since Jesus was tempted. But this skips over the nature of temptation: that a temptation in this fallen world is a situation-specific pressure that leads to sin when it meets with an internal corruption in our nature or character and successfully rouses desire for what is wrong. Jesus did not have the corruption of nature or character, and being tempted was not sinful because the external pressure to sin is not sinful. However, when we are tempted it simply demonstrates that we already have the tendency to sin. As one of the ancient Christian writers has said, "How well I know: temptation came because I wanted it."
I think we have to acknowledge that Jesus teaches that not only should we not set out to lust and that the intention is wrong, but also that lust itself is wrong whether we intended it or not. Lust is a form of coveting, which is wrong in and of itself; lack of intent does not remove the sinfulness of the wrong desires in our hearts.
Some people are pained that Jesus set some "almost impossible standards" for us. I think this is entirely understated. Jesus has set some impossible standards for us, and told us: "With man, this is impossible; with God all things are possible." If someone has an instinct to justify himself by keeping the law, he will be driven to despair by commands like loving our enemies, not lusting, and not coveting. This is a healthy thing, and the healthy response is not to blunt the force of the law, but to use that force of the law to accuse and convict the evil inside us, condemning our wrong desires and nailing them to the cross.