If a man commits rape and doesn’t consider it a violation of the law, it doesn’t change the fact that it is. Here is proof that lust is a violation of the Ten Commandments: “You have heard that it was said to those of old, ‘You shall not commit adultery’ [Seventh Commandment]. But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart” (Matt. 5:27,28). So there you have it from the highest authority on earth—the Word of the Living God. If you have the word of a king or a president, you have it on great authority. But this comes from the authority of the Word of God Himself. So, make sure that you quote the verse and don’t just refer to it. Jesus quoted the Scriptures word for word when He was tempted by the devil. God’s Word doesn’t return void. It is quick and powerful and cuts through to the marrow of a sinner’s bones, so quote it when you share the gospel.
Another way to strengthen the lust argument is to reason about it. Tell the sinner that lust is “pornography of the mind.” Ask if he consider child pornography to be okay. The odds are he will say it is morally wrong. That is his moral standard. God’s standard is higher than his, and He says that lusting after anyone other than your spouse is morally wrong.
Also, never forget that you have the sinner’s conscience on your side. It bears witness with the Law (see Rom. 2:15), and of course you have the help of God Himself, who promises that the Holy Spirit will convict of sin. So plant the seed and trust that God will faithfully cause it to grow.
Proverbs 6:25 Do not lust after her beauty in your heart, Nor let her allure you with her eyelids.