How would you feel if you discovered he assaults other women who for some reason are more vulnerable than you. Your friend may realise he is a predator (although not that he targets you) but she somehow copes with it to maintain her marriage.
I have every sympathy for you and am very sorry you are going through this. He is getting away with it because up until now you have let him. Perhaps by posting here you can take confidence from the views expressed to you. Your friend deserves better but it seems highly unlikely that your friendship would survive you reporting him.
You know that the choice is stark but if you do not report him he will carry on - there are currently no consequences for him. Have you ever challenged him or actually asked him to stop - in a way that sounds as though you mean it? By not doing so (and I realise how difficult it would be) you are, in his utterly twisted mind (although not in yours in any degree whatsoever), condoning what he does.
Because this has gone on for some time, if you do manage to challenge him verbally (and you clearly are not going to report him as others have suggested) he may claim that you 'like it' because you have not complained before. If you will not report the matter you have to take action - knee him in the scrotum, slap his face, dig your nails into his hand or whatever you can do at the time. You would then have to be prepared to argue that this was proportionate after years of abuse, entitlement, gross disloyalty to his wife.
In behaving as he does he is betraying your friend as well as you. You do not want that to carry on and you are the only one who can do anything about it.
So sorry you are going through this - I hope I do not sound harsh.