(Taken from the web series about masturbation)
Deliberately cultivating yearnings for anything that, if acted out, would be immoral, is as depraved as physically committing that act. For example, the thought might keep coming to you about relating sexually with someone you are not married to. Keep pushing that thought aside and your purity will remain intact. To intentionally develop the thought for your sensual enjoyment, however, is no less sinful than acting out your fantasy.
To understand this moral principle, consider Jesus’ teaching that hate is as bad as murder. Suppose a person is trying his hardest to shoot someone dead. He takes careful aim but to his great disappointment he misses and the person escapes unharmed. Does that make him more righteous than if he were a better shot and the bullet killed the man? The consequences for the victim would be vastly different but in both scenarios the sinfulness of the offender would be identical.
Let’s take this a step further. Suppose Phil and Sam hate Barry with equal venom. They both wish he were dead. If Phil could push a button, terminating Barry’s existence and be certain that no one would ever know, who did it, he’d do it. The one thing keeping Phil from murder is that he is too scared about the possible personal consequences (public humiliation and imprisonment) if he were caught. But Sam is braver and so kills Barry. Should we regard Phil as more moral, simply because he is the bigger coward? Obviously, Barry would be exceedingly better off if both men were cowards. Morally, however, both haters are equally corrupt, since both wanted him dead.
If you deliberately savor the thought, you want the sin as much as an impulsive person who acts out your daydreams. For anyone mentally engaging in immorality, the spiritual consequences are therefore as serious to the offender as if he had committed the physical act, even though the consequences to the victim are much less.