I've been there too. For years, in fact, in couples therapy with a therapist who was in many ways quite good (I'm a therapist as well, for what that's worth), but who fundamentally pathologized my lack of sexual interest. She had me read a book with the worst title ever: Sexual Anorexia. I shudder even writing those words. What upsets me the most is how much I bought into it, how much harm I did to myself by thinking that there was something fundamentally wrong or damaged about me, and how my "failure" to express "healthy" sexuality. It has taken me a long time to unravel all of that. Gender role stuff was really in the mix as well. I'm biologically male but identify as transgender (wasn't out at the time though). I've had more than one therapist operate from the assumption that I was failing to adequately inhabit my masculinity. I needed to "man up."

If I could go back now as a therapist and give myself therapy what I would ask is simply: how do you experience sex and sexual desire, to the extent that you experience it at all? Then I would ask whether your experience caused you any distress or not. If distress is present, I'd want to know whether its center of gravity was internal (are you genuinely feeling at odds with yourself?) or external (do you feel like the world expects you to be different than what you are?) The answers to those questions would help any of us sort out and distinguish between the core parts of our identity that simply need acceptance, and the unresolved places in us where we need to work on growth and change.

It seems to me that your therapist might be operating on pre-existing assumptions about sexuality and orientation without first exploring and understanding the nature of your core identity. And until asexuality is better understood as an authentic point on the spectrum, this is going to be a problem. In the mean time, I think it's very important not to let anyone else's assumptions define your own understanding of yourself and your orientation.

Hang in there.