"You shall not commit adultery."
When we remember that anyone who breaks any of these commandments is subject to death, we can begin to understand the seriousness of adultery. Why? Because adultery is a force so powerful and destructive that it can singlehandedly become grounds for divorce (Mt 5:31). Adultery destroys relationships. It takes two people who were bonded together and rips them apart, tearing at their hearts and souls and unleashed so much emotional pain and turmoil that it is an affront to God. No parent can stand to see their children heartbroken, and neither can God.
And yet, Jesus says that the one who looks at a woman lustfully commits adultery with her in his heart (same goes for women). Recognizing the destructive power of adultery, Jesus's solution is graphic: If your eyes look lustfully, cut them out. If your hand causes you to sin, (say by touching another person lustfully) cut it off and throw it away. You know why? Because it is easier to cut off one's own body parts than to correct a heart that lusts.
With man such correction is impossible. No matter how hard you try, you will never escape your sinfulness, but thank God he has sent us a savior! Now even when we sin, we have one who intercedes for us and defends us before the throne of God. It is Christ living in us that transforms us by the power of the Holy Spirit not to lust for only he can fulfill our every need. With Christ in us, what else is their to lust for?