Note 17 at Mt. 5:28: Sin doesn't just happen any more than children just happen. As stated in James 1:14-15, sin has to be conceived. First comes the thought and then the desire. Next comes the lust (inordinate or excessive desire). The act of sin follows. As long as we are in the world, we will be exposed to evil thoughts. However, 2 Corinthians 10:4-5 clearly states that the Lord has given each one of us spiritual weapons that can subdue and take every thought captive to obey Christ. We can limit our contact with sinful thoughts much more than most Christians have ever imagined. We do not have to let thoughts become lusts.

If evil thoughts are entertained, they become lusts and as Jesus is teaching here, we are, in the eyes of God, guilty of committing the act. Just having a thought is not sin. Enjoying, entertaining, and dwelling on that thought (which is lust) is sin.