Therapists are normal people, but specially trained to be open to diverse experiences and focus on understanding, caring and helping, not wasting time and energy on judgment.

I believe her saying she won't judge is a key reassurance, whereas her saying "you'd be surprised what I hear" leaves a lot of room for what you say to be "better, worse, less or more shocking." The point is... to her it's all relatively similar- she doesn't judge.

Since you're still fearful and her reassurance may not have been clear enough for you, perhaps you could just ask her: "If I tell you a sexual fantasy that I think is really disturbing, would you terminate our relationship if you found it too upsetting or disgusting." (Or fill in your choice of adjectives.) Or ask her how exactly she'd work through hearing something like that from you. My therapist has been very open and clear that we could work through any sexual material, including erotic transference and other specific uncomfortable scenarios. That was really relieving to me.