That's kind of my point though.
So, with my ex who I did experience this with, it was fairly casual. We weren't dating anyone else, but if one of us pulled on a night out, we weren't bothered. So, when she slept with someone else, it was because she wanted to sleep with someone else. The two times I watched while that happened, I really was just a fly on the wall. She was shagging him because she wanted to shag him, I was just along for the ride. And yes, she might have behaved slightly differently because I was there, but that wasn't the point.
With DP, that would never happen. I'm what I'd term as emotionally monogamous rather than sexually monogamous. While I only want to be in a relationship with one person, I could quite happily have sex outside of that relationship. I don't, because the person I've chosen to be with would not be up for that.
DP on the other hand is both emotionally and sexually monogamous. She only wants to be having sex with the person she's in a relationship with. So if she ever decided to make this fantasy a reality, then she'd by definition be doing it for me. It'd be an act. And that's not the point of the fantasy.
To give an equivalent that doesn't involve a third person -
DP likes having hot candle wax dripped on her. To be honest, dripping hot wax on her does nothing for me, but I do it because she likes it, and doing it is no skin off my nose. DP knows I'm not getting turned on by it, but that doesn't change the fact that she enjoys it.
DP would also enjoy tipping hot candle wax on me. We've tried it, and again, it does nothing for me. In this case, DPs enjoyment of it is reliant on me enjoying it, so because I don't get off on it, DP doesn't. I could put on an act and pretend it's doing something for me, but what's the point really. I wouldn't get anything from it, and DP probably would see through my act and it wouldn't turn her on either.
Its the same with my fantasy. It'd only turn me on if it turned her on.