You just don't know what it will be like ... a lot of my attraction to women shifted over time ... it turns out being a repressed woman made me react to women with a kind of obsessive reverence that diminished once I became a woman - but the changes were not a complete reversal into only wanting to date men ....

I know some trans women feel more like women when they date straight men, and that's a real psychology, but even though I'm bi and now attracted to men, and I would feel validation as a woman by dating a straight man, I still wouldn't do that (even if I were single).

Like you I just love and trust women, I could see sex with men but a relationship would probably be harder to make happen. Even with my most sensitive queer male friends I struggle in the friendship in ways I don't with women - I'm just not cut out to date men, I want too much emotional intimacy and feel too much like a lesbian if that makes sense 😅

Anyway, you should talk to your partner, getting lost about the details of sexuality changes on HRT misses the most important part, which is that they are feeling vulnerable and insecure, and reminding them that you will love them regardless of how your sexuality shifts could be helpful.

edit: (and in my experience, you might feel vulnerable and scared of losing her, too, as you transition - my partner is pan and very supportive of my transition, but I still felt scared and vulnerable when I transitioned, they are huge changes and it makes sense both people might worry the relationship could be impacted or romantic interest might not be the same as it was, etc. - I certainly became a very different sexual partner, but in the end it worked for my spouse and she is very happy with my transition)

edit2: also might be worth exploring why it matters to your partner whether you experience attraction to men, I keep forgetting how common it is for cis straight people to have misconceptions about gender and sexuality, she might fear your gender stuff is actually repressed homosexuality, it's also possible she doesn't believe bisexuality is real and that if you experience attraction to men that you will really be a gay man rather than a bi woman - might be worth getting clarity on what the beliefs and concerns are on her end.