If you've done it already, it's OK to do it; or, two sins for the price of one?
Jesus taught that those who lust are adulterers in their hearts and those who hate are murderers. Some have taught (especially in the former case) that if it is in your heart you'd just as well do it.
Matthew 5:28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
I have heard this verse used -- either confusedly or deliberately -- to justify committing adultery. The thought process is that is lusted against a woman you have committed adultery. So, since you are already guilty of the act, you just as well go ahead and commit the act physically. The problem is that one sin is not justification for another. If you lust after a woman you have committed adultery with her in your heart. One sin. If you commit adultery with her in your body, you have added a second sin. The first requires repentance, not the piling of a second sin upon the other. If you've committed one sin, it does not make it OK to commit a second sin.
1 John 3:15 Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
If we applied the former logic applied to the case of lust to this verse, then if someone hates his brother he should just go ahead and kill him. I think most see the vacuity of such logic -- though some have probably tried it. Obviously, the thing to do if you hate your brother is to repent, not go ahead and kill him. If you've committed the one sin, it does not justify the second sin.
Lust being adultery and hate being murder is very real. BUT it is not justification to follow up on the secret mental sin with the physical act.