He also felt bad for the OW, that she was all alone, etc. I kept gently reinforcing the point that it was also best for her, she could move on. I totally agree if you think you have made your case already. Nagging gets us nowhere. But if you don't think you've really made the point that it is impossible for him to have what he desires with you this way, give it a try.
Men often relate better to facts in written form.
It's not just you saying this. Some here have the idea that it is a LB to educate your spouse. It does no good to lecture them, especially when in the fog. But all life is educating, negotiating. I've always believed it's how you do it.