Well, good relationships are basically partnerships, which are built on a foundation of trust, integrity, and communication. If the only reason you want this is because you're afraid she would cheat and do it anyway if you said no, then you need to focus on the foundation of the relationship. What it seems you're doing is encouraging your current GF to eventually follow the footsteps of your X. Its interesting that often divorcees tend to get involved a second & third time with partners that are similar to the one that just failed, thinking that this time it will be different, and it often isn't.
The reason I suggested a Dom/Sub group is because that kind of activitiy feeds many of your needs, but has a on/off switch when not attending their functions, rather than a relationship as you had before where things were randomly happening with little or no control by you. My point being, it puts a parameter around the activities.
I would be asking myself ... why would she cheat on me? ... what can I do to discourage her from cheating on me? And if you discover there's really nothing you can do that would discourage her from cheating, and her cheating is a discomfort to you, then you get out of the relationship. Life is "fucking short" my man!
Sometimes its good to just look at the reality of things ... it seems you may be struggling a bit with your own sexuality (based on past comments) ... and it could be adding frustration to you. Making a relationship work is difficult in normal, monogomous situations, and what you desire is very complicated. Thus you need a partner that's understanding and going to work with you, or you're going to fail again. Good luck! Mac