Alex, you seem to be on the right track here. I know it's hard to not push forward at this time, especially since you are now seeing her beginning to open up and become more interested. Now would probably be the worst time to press anything. If I'm shopping at a store, I'm a LOT less likely to buy something if the salesman is pasting on his biggest chiclet grin and hovering over me like a vulture. Same sort of idea with this. I think that what you've described in your post is actually a 5 year conversation, and what she's saying right now is, "OK you've got me. So I'm interested. I suppose you're going to get all pushy now, right?" You are telling her that you're not, and your continued restraint is only going to reinforce what you're telling her. The goal at this point is simply to be completely approachable and supportive. This is a big step for her. You're not just asking her to take her clothes off in front of someone else; you're asking her to make herself more completely vulnerable than she's ever been before. It takes a lot of trust to show one's true self like that. What she normally shows you is the woman she thinks you want to see. What you're asking is for her to show you who she is without you, who she is with someone else. This is a part of ourselves that we guard very closely because it's something about ourselves that we cannot change. To open that part of ourselves up for possible criticism is very risky.
I would just stop beating around the bush and lay it out on the table for her. You would say something like, "Look, I just want to see you happy and living life to the fullest. I know that fantasy and reality are two different things, but if you ever want to explore the possibility of making your MFM fantasy a reality, all you have to do is ask. I'm not going to be angry or stop loving you or lose respect for you just because you happen to like sex! I love that you're a total sex goddess! All I ask is that you don't try to hide anything from me. I just want honesty."
What you want her to understand above all else is that you value her, and you care about her wellbeing. This means, of course, that you would never ask her to do something she wasn't ready for or something that made her feel badly about herself. If that's how she feels, then just stick with the fantasy.
I'd also like to add that if you do get started, do not try separate rooms or kiss and tell type dates. If she's too uncomfortable to have you in the room, she's not ready for it!! You don't have to go all out right out of the gate, you know. You can try milder situations where the clothes stay on. Ask her to point out some guy she's attracted to and tell you what she likes about him. Ask her to tell you what she's thinking while she watches him. Maybe try a situation where she can flirt with someone, or dance with him. Just play around with the idea, and find safe ways to bring the fantasy into the real world.
Good luck!