Tell him that you have realized that the only thing you might get from this guy is an orgasm, and that you can do that yourself, and even take yourself to dinner in public after you do it!!! All you will get from him is laid. You will not take the place of or have more value than his wife. If you think you will, remember that, regardless of what is going on in the marriage today, at some point he looked deeply into her eyes and took her as his wife, and now he is disregarding her dignity by coming on to you. He will devalue your relationship and do the same to you - entertain that as fantasy, and get far away. He should have enough character to leave the marriage rather than play people for fools.
As a mistress you will not go to dinner, movies, Vegas, meet his or your family. You will be relegated to being a secret, a deception and a lie. That will be your status. All so he can try a little "strange" on the side.
I don't care that his wife treats him bad (classic bs line) or that religious reasons prevent divorce (more bs) or that she has him by the financial balls - or that he looks like George Clooney, you cannot prosper in a situation born of spiritual deceit. Tell him to go away. Find someone else to flirt with.