I think you should "practice" here, what sorts of things you would say/how you would bring them up. It's a fantasy/dream sort of discussion, not a whole lot different from what would happen if we went for a drive with our T's, or they came and stayed the weekend, etc.

Have you dreamed about your T and told him the dream? Talking about sex, even fantasies about him, can be like that; where are you located, is it just about "sex" or is there a whole action fantasy (like people thinking about "suicide" versus those with an actual plan), what are you wearing -- get at the symbolism rather than the "act" itself which is probably the whole point?

I once dreamed I was sucking my own breast and told my T the dream and she instantly understood it to mean I was trying to nourish myself (instead of letting anyone else/her help/feed me) and it was just so "right" and I wouldn't have thought of it in a million years because I was hung up on the embarrassing aspects of the image and having to tell her, etc. Treat anything embarrassing as the "fantasy" it is or as if it were a fantasy and attack it "intellectually"/third person.