Welcome to the board, Starla! Glad you've found us. As you can see so far, there's not much support here for the likes of your boyfriend. I personally like goodtimes suggestion about tattooing his forehead (heh heh heh), but while it's a nice fantasy, I don't think you should actually do it.
Western Swing is right: walk away and let him know why you're leaving. Don't yell or get shrill, just look him dead in the eye and tell him you know who and what he is, and he is not the kind of person you want in your life. Let him know you are ashamed to be with him and that you are very disappointed in him. It's the worst thing you could possibly say to him. Then just walk away. His and his wife's marriage is none of your business although he's insisted upon making it your business. That being the case, don't be sucked in. He'll show his colours soon enough.