I've told L about some of my fantasies (some about her, some not). She handled it extremely well. For me, they're just thoughts, not desires.

It sounds like your fantasies are desires? I think if you have a good therapist, she'll handle discussing fantasies well.

The fact that you want to ask her for your fantasy sounds extremely inappropriate no matter how your therapist reacts. Also, you asking here how she'd react, shows that you know this would be inappropriate.

I don't know what you hope to gain by doing this. You know the answer is no, so why ask? I think it would be healthier, more appropriate to talk about your fantasies, not asking for it.