It is not always only outward behavior that is sin. Sexual lust is also sinful in the sight of God. God gives people the ability to think and to control their thoughts and desires. The option to think and desire either good or evil is very clear in the Greek text, because most words for “desire” can describe either good or bad desires. It is the context that determines the nature of the desire. Thus, in most versions, orexis is translated “lust” in Romans 1:27: “…men also abandoned natural relations with women and were inflamed with lust (orexis) for one another.” Spiros Zodhiates defines orexis as: “Appetite, lust or concupiscence. It is always the reaching out after an object with the purpose of drawing it to oneself and appropriating it.” 
Another Greek word translated “lust” in many contexts is epithumia. Although epithumia often refers to desires that are godly, it more often in the New Testament refers to desires that are ungodly, which fits with the fallen nature of man. Some of the ways it is translated in the New Testament are “lust,” “evil desires,” “cravings,” “sinful desires,” “passions,” etc. Titus 2:12 says that the grace of God “teaches us to say ‘No’ to ungodliness and worldly passions (epithumia).” Christ said that if a man looks lustfully at a woman, he has already committed adultery with her in his heart (Matt. 5:28). 
The Christian is not to let sexual lust run amok in his mind, but is to bring his thoughts captive to Christ.
 Spiros Zodhiates, The Complete Word Study Dictionary (AMG Publishers, Chattanooga, TN, 1992), p. 1057.
 This verse is very confusing to many people. First, “lust” is “excessive, unrestrained desire.” It is not being lustful to notice that a woman is attractive. It is being lustful to “take her to bed in your mind.” People often wonder why looking with lust is equated with the act of adultery itself. The reason is that if a person will commit adultery in his mind but not with his body, it is usually because of a fear of consequences or simply his inability to make it happen. God does not want people to obey Him only because they are afraid of consequences. He “looks on the heart” and wants Christians to get to the point that they obey Him because they have internalized His Word and thereby have pure hearts that do not desire to sin.