The phrase "to lust for her" means to desire her (or him) sexually and causes the adultery which takes place mentally and in the heart.
Proverbs 6:25 says, Do not lust after her beauty in your heart, Nor let her allure you with her eyelids.
Adultery falls under sexually immorality. A look at 1 Corinthians 6:18-19 states,
Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body. 19Or do you know that your body is the temple of the Holy Spirit who is in you, whom you have from God, and you are not your own?
Since we are a temple of the Holy Spirit, adultery is a sin that not only breaks one of the ten commandments of the Old Testament, but we need to remember that sin defiles that spiritual temple.
29"If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. And if your your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell."
The message behind this passage is simple. Get rid of all temptations to evil no matter the costs. If we continue to live by the flesh, giving into the flesh, and not seeking change to the life we are called to live, then hell will be our reward.
Let's also look at Colossians 3:5, 5Therefore put to death your members which are on the earth: fornication, uncleanness, passion, evil desire, and covetousness, which is idolatry. Because of these things, the wrath of God is coming upon the sons of disobedience, in which you yourselves once walked when you lived in them.
The warning is clear.
