From A to B

My wife and I have a great relationship. We have a solid foundation built on friendship and trust. Over time we have played around with a little bit of Femdom, but nothing too crazy. She knows I fantasize. A lot. Incessantly. I've told her once or twice that the thought of seeing her with another guy is a turn-on. But where I kind of hit a roadblock is trying to convey to her how much of a turn on it is, and that it's a serious fantasy, not just something I want to kick around in my head. So my question is how do I take the conversation to that next level?