I am against the medicalization of gender identities on principle because I consider it harmful. If a mature adult wants to do it to themselves and is prepared to pay for it, then fine. However, it is not for under-25s and not for the rest of us to pay for. Simple enough? Plus...

Any mature adult pursuing medicalization of their gender identity cannot expect the rest of society to treat their gender identity as if it were a sex. Sex and gender identity are categorically different. Furthermore...

It is entirely inappropriate to expect society to consider gender identity to be something worthy of protection to the same extent as sex, race, disability or sexual orientation.

Sex is innate.
Race is innate.
Disability is real and measurable.
Sexual orientation is innate.
Gender identity is a feeling (see the official UN definition if you don't believe me: https://www.unfe.org/definitions/ ).