[Rain Man]

Might I suggest that you use a strategy of negotiation? Perhaps you can tell her that you will indulge her fantasy and follow all of her wishes, but the next day she needs to do the same for you.

When she asks, start with something reasonably acceptable like, "Do the anal thing again, but from [pick a position]. And there are a few more details, minor stuff."

When she asks about the details, start her with something relatively common like, "You're wearing handcuffs". Plus a few other details.

Keep her going, and keep getting more outrageous, but do it slowly. See how far you can get her to agree before she thinks you're the most disgusting pervert who ever walked the planet. I will give you rep if you can get her to Level 9, which involves, among other things, being observed by a group of masturbating women in burqas. And a few more details.