Tammy,
I don't know a single person who has negotiated that type of relationship. I'm guessing there is usually one partner willing/wanting and the other not so much...or violently opposed. My wife is one who is very opposed.
BTW...I just checked out your book on Amazon and I'm thinking about reading it with my wife. She has a difficult time talking about sex with me...and I'm thinking your book can help.
I am wondering what you think, what you are doing and how you are negotiating fidelity in your relationships???