Most Christians are familiar with the Apostle's Creed. It contains an affirmation which OCDers seem to have a great deal of difficulty with: "I believe in the...forgiveness of sins."
"Is this a sin?" "Is that a sin?" The anxiety with which some Christians ask that question about the minutia of their lives seems to call into question whether they really do believe that God forgives sins! The fact of the matter is that we can't go through a single day without sinning. In fact, we commit many of our sins without even realizing it!
Fortunately, we are justified (declared righteous) neither by maintaining a clean slate nor (as other OCDers seem to think) by the good work of confessing each individual sin and asking for forgiveness. We are justified by grace, for Christ's sake, through faith. A believer is judged perfect by God even when she commits a sin of weakness of which she is not aware, and even before she confesses it to God and asks for forgiveness! We are declared righteous simply because Jesus died for us, and we grasp hold of that gift simply by believing!
We don't confess or even ask for forgiveness so much in order to be forgiven, but because we are forgiven. We want to apologize to God in much the same way we would apologize to any other friend whom we had done something to offend, even if we knew that he didn't hold it against us. But we are under no compulsion to dot every 'i' and cross every 't.' God forgives us by grace, and we receive that forgiveness through faith.
In fact, don't look now, but God forgave every sin you will ever commit the moment Jesus cried "It is finished!" from the cross. Even faith simply lays hold on what He has already obtained for us.
A person who is grateful for that gift will not blithely sin away without worrying about it. In fact, he can't; the Holy Spirit who dwells within his heart won't let him. And certainly Christians strive to think and to do those things which are pleasing to God. But even sinful thoughts need not be repressed or magically undone; once a thought has passed through our minds, all that remains is to repent and move on. Better, surely, to do that than to keep a sinful thought in our minds by dwelling on it and worrying about it!
And if it's not sinful, even that doesn't have to be done.
So why in the world would a person who believes in the forgiveness of sins worry and fret about whether this or that thought was a sin? When it comes down to it, all of our thoughts, in themselves, are soiled with our sinfulness. Best to simply commend our thoughts- especially the ones we're tempted to worry about- to God's grace, and move on, joyfully trusting in the forgiveness of sins.