Say to her, "I know it is a fantasy, but could we play it like a tease game and pretend you are having sex with several guys that we know. If you were doing that, which guy do we know that you want to be naked with first?"
If you spend a year on the tease game, a point will come where her imagination grows to crave reality to allow it to happen. BUT PLEASE BE PATIENT, do not force her, just have fun and don't talk to her about making it happen soon.
Feed her mind and if she enjoys the game, she will eventually give you the chance to live your fantasy. Don't flirt with other women (remain monogamous) as this causes her to think you have secret agendas.