Hmmm. This is a tough one. I think, maybe she's just making sure there isn't something "deep down" that you're suppressing (ie, an attraction to men).
I'm Bisexual, and my therapist is a Lesbian. She has asked me (on more than one occasion) if I think my Marriage problems are due to the fact that I married a man.
I came right out one day and asked "So, are you trying to figure out if I'm actually a "hardcore lesbian" and not bisexual?"
She laughed and seemed kind of surprised, and said "No, I was just "digging" a little, that's what therapists do."
She has also asked me which sex I tend to be more attracted to - emotionally, and physically. Again, more than once.
At first, I thought maybe she was trying to tell me that I'm actually a "full lesbian" (for lack of a better term) and not actually bisexual. Especially when I told her that I tend to lean more towards being attracted to women. (Obviously, not always, I married a man lol)
So, I confronted her about it, and again, she said that they were just questions. Therapists are trained to dig into our psyche. They can't always know if we're supressing something, or denying something. There are even times when there is something deep down that we aren't aware of. They like to make sure there isn't something that needs to be "pulled to the surface".
That being said, your T could be bias, she is human after all....so I think it would be in your best interest to ask.
Maybe start with something like "So, you asked me if I had ever slept with a man before...I'm just wondering what your thought process was behind that question..."
Good luck! And keep us updated!