"Why does my wife call me an adulterer? Sure, during the salsa dance, I asked her hot sister to sneak away with me to make love, but I struck out. I never seduced my sister-in-law."
Jesus Christ warned about adultery in the heart:
 "You have heard that it was said, 'You shall not commit adultery.'  But I say to you that everyone who looks at a woman with lust has already committed adultery with her in his heart.  If your right eye causes you to sin, tear it out and throw it away; it is better for you to lose one of your members than for your whole body to be thrown into hell.  And if your right hand causes you to sin, cut it off and throw it away; it is better for you to lose one of your members than for your whole body to go into hell. (Matthew 5:27–5:30)I heard many people misinterpret the above verses from Christ in the Sermon on the Mount. The faulty interpreters suggest that Christ condemned all sexual temptation when he warned about lustful looks at beautiful women. But the same Gospel that warns against lustful looks also says in Matthew 4:1–11 that Christ resisted temptation. Likewise, mere thoughts and feelings of temptations are not sinful. In the case of lustful looks, those looks go beyond temptation in the sight of sexually appealing eye candy.
The warning against lustful looks goes back to the tenth commandment in Exodus 20:17. That commandment warns men that they shall not covet their neighbor's house, wife, or anything that belongs to their neighbor. And the word "covet" refers to an excessive desire.
For example, in a modern day scenario, a man named John might see that his next-door neighbors own a stunning house and the wife looks stunning while she dives into her pool or applies sunscreen. This might instigate John to feel temptation. He might feel tempted to takeover his neighbors' house and make love to the wife. At this point, John has not coveted the house or wife. And if he refuses to harbor the temptation while following the Lord, then he avoids the sin of coveting.
But if John decides to feed these temptations, then he begins excessive desires for forbidden possessions. He might begin to make plans. He sees no way to steal the house, but he plans to seduce the wife. When opportunity arises, he tries to make the wife feel special. After developing a rapport with her, he subtly makes suggestive glances while trying to seduce her. He tries to pace the seduction over a few days or even a few months, but nonetheless, John commits adultery in his heart.
Regardless of how far John goes with his adulterous plans, God calls him to repentance and restoration in the name of Christ.
Copyright © 2012 James Edward Goetz
New Revised Standard Version Bible, copyright 1989, Division of Christian Education of the National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved.