Unfortunately, there's little you can actively do to convince your daughter that she's making bad decisions. If I were you, I wouldn't let this married man come to any family functions or come to your house. You don't have to entertain people, in your own home, who you disapprove of. As for your daughter, I know that, right now, she thinks things are just fine. But, as a woman, I know that it's very difficult to love someone who will never consider marrying me. The whole "dating a married man" (whether he and his wife have an open marriage or not), thing is a recipe for heartache and pain. I know your daughter doesn't realize this now and she may have to learn the hard way that these situations are very, very rarely good for anyone. I know how hard this must be for you. You certainly should feel free to make your displeasure known, but I wouldn't cut her off from contact with you, because she needs a mother who has some sense and loves her. She'll really need you when the whole thing blows up on her one day. Then, you will be there to help her pick up the pieces of her shattered dreams and, hopefully, she will have learned a very hard lesson about life and love.