Hi, Happy to try and answer, though with the caveat that this is just from my own experience.
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Disphoria happens on a social, mental, and physical level. For some the social hits strongest, basically they feel like their true gender internally and would like to act and be treated in a way that matches the social contruct of their true gender rather than their AAB gender, but they may care less or not at all about the physical appearance. Transmedicalism denies that their experience is very much the trans experience. For some, like me, my lifelong dismorohia (e.g. eat disorder, body shame) meant that my physical disphoria only presented after I found queer communities will to accept me as trans while I still looked very cis. To put it in TLDR terms, no woman should have to shave her legs to be a woman (and trans women are women). Which leads me to #2.
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Transitioning can take a long time. I am now addressing my physical disphoria, but the time and money required is significant. Transmedicalism (perhaps unintentionally) creates hierarchies: passing > surgeries > hormones etc. Which can be emotionally destroying along the way, and even more for a late-bloomer like me, who may never pass. And so rejecting that helps change the standard for me, makes sure every step I take is for me which leads to the last point.
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NB and trans NB people exist. Specifically why what OPs dad said is still troubling, it still forces a binary when gender is more of a 4d spectrum.
TL;DR transmedicalism gatekeeps a lot of people out, when we all have far more in common.
Again, not the gospel, just my take. Thanks for asking. ❤️
"God, what have you done!"