Morally, there is a huge difference between sin and mere temptation (even the holy Son
of God was severely tempted) but Satan loves to muddy the difference. He tempts by putting
evil thoughts in your mind. One of his ugliest tricks is to then make you feel guilty about
his own failed attempts to tempt you! If sinful thoughts keep coming and you keep
fighting them, God commends you for the fight and regards you as victorious. The thoughts
are originating not from you, but Satan. It's enjoying wrong thoughts –
deliberately entertaining them – that is wrong.
If Satan succeeded in actually getting you to sin in your mind he would then try the lie
that you have blown it so much that you might as well sin physically. However, no matter how
much mental sin corrupts the fantasizer, it leaves the other person pure. To sin with
someone else doubles the evil because it corrupts the other person as well.