Not having read the full thread I cannot see how it isn't a mental disorder.
Being gay is not a mental disease because it is within the normal range of sexual expression for humans. If you believe that sexual intercourse and by extension sexual attraction is purely for reproduction I guess you can make the case. Most people having sex are doing it for a myriad of other reasons.
Trans-sexuality and Transvestite-ism have been lumped together, and now there is this new trans-lite that is affecting only GenZ and a few millenials.
Transsexuality was classed as a mental disorder because it was persistant and disabilng to the individual. From what I can understand it is as a result of severe and traumatising internalised homophobia/misoginy/misandry and primarily effects gender-non-conforming same sex attracted children. Thats is why gender reassignment was first started because it was resistant to talking and behavioural therapies i.e. they couldn't get rid of the extreme and disabiling discomfort.
Transvestisim is another kettle of fish. From what I understand the root cause is a trauma to a dominating female care giver as a little boy. Sometimes that trauma becomes sexualised i.e. a fetish and sometimes it doesn't resulting in non-fetishistic trans vestitism. Most of this can be managed by the individual and I don't think it is a good thing to shame it -as it is a trauma response.
The new non-binary/transmasc/fem is a trend predominately effecting outsiders whether they are neuro-atypical/same-sex attracted/sexual assualt-survivors etc. These people would have had a mental health disorder/addiction issues anyway and this is just the newest version. In the past they would have been punks, emos and goths. This is the first generation whose adolescence is completely defined by social media and the internet rather than music.
Where I think the issue is, is with the tolerance of the narcissim that is present in some of these individuals coupled with internalised/externalised misogyny. We all know that activists in general are pretty narcissistic individuals and enjoy dominating conversations, peoples lives because they simply do not have the capacity for empathy. It would be interesting to know whether trans-people have a higher level of narcisisstic behaviour and higher incidence of NPD than the general population or if that is just an impression I have from the fact that people with higher narcissim tendancies are popular on social media.