At 17, I was forced to get therapy for other problems. I didn't want to be there, and didn't want to talk about anything (let alone gender). At the end of the first session, the therapist called my mother in and asked me to sign a paper for her to talk to my mother. She asked me to sign the paper while my mother was in the room. My mother is controlling and verbally crap, so I signed it because I didn't want an argument. I lied to that therapist from then on just to get out of therapy, but later learned that the therapist shouldn't have done that.
Depending on your age and where you live (I'm in the US), some places won't allow a parent to know what happens during therapy without your consent. I would look up the rules for your country or state if you aren't sure.
If you are already in therapy and are alright with your therapist talking to your dad, then great. Similar to what Dena said, a good therapist will keep what your dad says as just a perspective from your dad, not any way with how you are. Worst case, if your therapist isn't acting how they should, you can always find a new therapist. Good luck.