Mathews 6:27-28 You have heard the commandment that says, ‘You must not commit adultery.’ 28But I say, anyone who even looks at a woman with lust has already committed adultery with her in his heart.
The Old Testament law said that it is wrong for a person to have sex with someone other than his or her spouse (Exodus 20: 14). But Jesus said that the desire to have sex with someone other than your spouse is mental adultery and thus sin. Jesus emphasized that if the act is wrong, then so is the intention. To be faithful to your spouse with your body but not your mind is to break the trust so vital to a strong marriage. Jesus is not condemning natural interest in the opposite sex or even healthy sexual desire but the deliberate and repeated filling of one’s mind with fantasies that would be evil if acted out.
Some think that if lustful thoughts are sin, why shouldn’t a person go ahead and do the lustful actions, too?
Acting out sinful desires is harmful in several ways: (1) It causes people to excuse sin rather than to stop sinning; (2) it destroys marriages; (3) it is deliberate rebellion against God’s Word; (4) it always hurts someone else in addition to the sinner. Sinful actions are more dangerous than sinful desires, and that is why desires should not be acted out. Nevertheless, sinful desires are just as damaging to obedience. Left unchecked, wrong desires will result in wrong actions and turn people away from God.
Under Gods command 