Jesus said, “You shall not commit adultery” (Matt. 5:27). The word, “adultery” (moicheuo), means, “literally, of sexual intercourse with someone who is married to another.” (Friberg) In the NT, adultery refers to a married man who has sex with a married woman who is not his wife.
Let us clarify the difference between adultery and sexual immorality. In the NT, sexual relations between an unmarried man and an unmarried woman is called “fornication,” or “sexual immorality.” But sexual relations between either an unmarried or married man with someone who is married to another is called “adultery.” (Louw-Nida)
The Law of the LORD says, “You shall not commit adultery” (Ex. 20:14). You shall not have sex with the wife of another man. You shall not have sex with the husband of another woman. That is adultery. You have heard it said, Jesus said, You shall not commit adultery. This is the traditional teaching based on the Law of the Lord. “You shall not commit adultery.” Jesus affirms that.
Yet Jesus said, “But I say to you.” Jesus now interprets this Law in light of the kingdom of God. Jesus does not challenge their teaching. But He challenges their hearts. (Keener) Jesus challenges our hearts today. We might believe the right teachings. But do we obey it?
“But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart” (Matt. 5:28). Jesus shows us that adultery not just an act, but an attitude of lust in the heart. Jesus is saying that even if you do not commit physical adultery, but you desire to do so in your heart, you have already committed adultery in your heart. (Keener)