TEMPTATION IS NOT AN ACT OF SIN
A good example of willful sin would be Adultery, which is an act of sin that occurs when a person chooses to set aside God's law and have sexual relations outside of marriage. Please take note that to commit Adultery is an act of sin, but the lust which precedes the decision is NOT AN ACT OF SIN, rather it is the manifestation of the Sin nature.
"But I say unto you, That whosoever looketh on a woman TO lust after her hath committed adultery with her already in his heart." Matthew 5:28
Jesus was saying this: "The Pharisees have told you that you are committing Adultery only if you actually have relations with a woman, but I say to you that it is the intent of your heart that God judges. If you look upon a woman lusting to have sex with her, and the only thing stopping you is the lack of opportunity, then you have already broken God's law in your heart."
The word TO means: "indicating intention, purpose or end." (Websters)
If a man is looking on a woman with the intention of lusting after her then he certainly is not trying to avoid lust! He may be sure that his heart is still set upon evil. He may also be sure that if he would commit Adultery given the opportunity, he is not truly Born-Again because John says that "Whosoever is born of God doth not commit sin." (1 John 3:9)
Looking upon someone TO lust after them is a sin, but the Devil wants Christians to think that just looking at the opposite sex and feeling desire is also an act of sin. That is not true! It is simply the first stage of temptation. Temptation to commit sin does come through lustful desires, fantasies, etc. but those lusts are not in themselves an act of sin.
If lust (desire) itself is an act of sin then Jesus committed sin because the Bible says that "every man is TEMPTED when he is drawn away in his own LUST," and that Jesus was TEMPTED like as we are,".
Being tempted is not an act of sin....