The Greek word translated as "looks" implies intent or special contemplation. The word underneath "lust" means "to set one's heart upon" or "to long for." Taken together, they make Jesus' instruction obvious.
Evidence from other portions of God's Word shows that it is not wrong to desire a husband or wife lawfully, but it is most definitely wrong when the one desired is legitimately beyond the reach of the admirer. How often does such admiration merge into a desire to possess and thus break the commandment? Considering the national statistics on divorce, this must happen frequently.
The Jews of Jesus' time perceived adultery as a kind of theft. This is not entirely wrong, but in this context, Jesus' emphasis is on moral purity: Ruin awaits those who are unchaste, even in thought.
Perhaps nowhere in Scripture is the inwardness of Christ's teaching as evident as with this commandment—inward in the sense that within is where sin begins and also where change must take place. It identifies where the problem resides. Christ carries impurity back beyond the lustful act to the first touch of the hands to the look of the eyes—and beyond these, to the first inception of desire. The Christian must "amputate" the desire so that the sin never becomes an act. We will remain pure and so will the object of our desire.
God gave us the wonderful gift of imagination, but if fed dirt by the eye, the imagination will be filthy. Sin begins with our allowing the imagination to dwell on what it should not. What feeds the imagination is so very important to moral purity and thinking and therefore to sin. Philippians 4:8 provides excellent insight:
Finally, brethren, whatever things are true, whatever things are noble, whatever things are just, whatever things are pure, whatever things are lovely, whatever things are of good report, if there is any virtue and if there is anything praiseworthy—meditate on these things.
The advice is clear: We must stop feeding our imagination dirt. We have to deal radically with sin! The purpose of this discipline is enrichment of life. The person who is condemned here is the one who deliberately uses his eyes and mind to awaken his lust so that desire is stimulated.
It is hard enough to avoid lusting after natural things, but this world deliberately designs many things to spark wrong desires in us. If certain books, pictures, magazines, places, activities, or people cause temptation, they must be avoided regardless of the cost. Avoiding sin is that important!