Your thoughts and curiosity is very common, especially in regards to therapy situations when clients have experienced an intense transference to their therapist, which is your case, as I understand.

Beyond that, I don't think anyone would be able to tell you the exact specific reasons why this is happening to you. It might have something to do with the fact that your sex life has been virtually non-existent up until this point, as you said. But this is much more than just that.

Sexual energy is, arguably, the most powerful force in our mind-body system, may be after maternal protective instinct, and, unfortunately, the most misunderstood one. People, in general, reduce it just to physical sexual acts, sexual pleasure and a means for procreation and thus overlook how it can and often is unconsciously used as a tool of power and control.

Those, who are more developed see the constructive side of it as an integral part of emotional intimacy in romantic partnerships and don't separate it from the emotional closeness. And, the tiny minority of the most evolved people understand its creative power and channel it to create art, music, social changes and anything that becomes their Self-expression and leaves their imprint in the world a.k.a "legacy".

So, sexual energy is a huge thing that has a powerful hold on our unconscious and subsequently our behavior. And, because of that, it's very unfortunate that we are completely in the dark about this subject. People are afraid to really "go there" and explore it because it goes so deep that it can expose the deepest human vulnerabilities and dark impulses the society doesn't like to see exposed.

Sorry for this long, philosophical answer. All I wanted to say really is just that you won't find an answer to your question from the outside sources. If you want to understand what makes you be curious about someone else's sexuality or to understand the role the sexual energy played in your life and how it affected your personality and behavior, you'll need to look back at the environment you were raised in and the attitude and the unspoken, as well as spoken, messages about sex you received there. It's a deep analytic work that could valuable insight for you if you do that.