It's a couple of issues - particularly in Canada - neither of which has anything to do with an adult's ability to make their own decisions about their own life/body.
1. Controlled Speech:
“addressing [Maxine] by his birth name; referring to [Maxine] as a girl or with female pronouns whether to him directly or to third parties; shall be considered to be family violence under s. 38 of the Family Law Act.”
2. Non-Adult Medical Decision-Making:
To summarize, there's a risk that a significant amount of young-women may mistakenly choose permanent, life-altering, irreversible damage and have the ability to do so without the consent of a parent, or proper advisement from a medical professional. Context/details below:
The Concern:
It seems to be, partially, a social phenomenon: There's an epidemic amongst teenage girls. Traditional gender dysphoria usually begins in early childhood (4-5). It used to be mostly young boys. It's shifted to 70% adolescent girls, a rise of 4400% from the previous decade (based on data in the UK). 65% of adolescent girls who have discovered transgender identify (without a prior history of it) have done so after a prolonged social media exposure. Trans identification is clustered in friend groups at 70x the expected rate. These statistics seem to indicate there's more to it than an environment of acceptance. We've yet to see a wave of adults come forward looking to transition affirming the current ratios among teens.
The Risk:
"When you stopped puberty with puberty blockers and go straight to cross-sex hormones, you absolutely guarantee that you will be infertile. When the gender clinicians pushed the parent to start her pre-teen child on hormone blockers, they were proposing that she put the child on a path towards infertility. What's more, even if her daughter did not start puberty blockers and instead waited puberty out and then started cross sex hormones, testosterone, this started all sort of risks of its own: endometrial and ovarian cancer, hysterectomy."
This, of course, doesn't account for breast-removal, permanent facial hair (even if testosterone is halted), or other side effects.
"a leaked 2019 report from the Tavistock & Portman Trust Gender Clinic in the UK, [...] showed that rates in self harm and suicidality did not decrease even after puberty suppression for adolescent natal girls. The report was so damning that a governor of the clinic, Dr. Marcus Evans, resigned. He told the press that he feared the clinic was fast tracking youths to transition to no good effect and, in some cases, to their harm."
The Decision-Maker:
Medical professionals can be discredited for doing anything other than immediately affirming the belief of the adolescent:
"But the Affirmative Care Standard, which chooses between these diagnoses before a patient has even been examined, has been adopted by nearly every medical accrediting organization: [...] health professionals can assist gender dysphoric individuals with affirming their gender identity, exploring different options for expressing that identity, and making decisions about medical treatment options for alleviating dysphoria. Notice whose medical judgment is in the driver's seat. Hint: It isn't the doctors."
Currently, adolescents are able to change their identities at High School without knowledge of the parent. (I don't recall if any of this applies to Middle Schools, too.) Soon, they'll be able to go get testosterone treatments, without parental consent or knowledge, during school hours. Eventually, they may even be able to do so at on-campus clinics.
"Hay" Comparison:
"We know that homosexuality can't be eliminated through socialization. [...] We don't have any similar weight of history arguing that we can't treat gender dysphoria."
"Before gender therapy was in vogue, gender therapists practiced watchful waiting, therapeutic process whose goal was to help a child grow more comfortable in his or her biological sex. [W]atchful waiting was remarkably successful. Several studies indicate that nearly 70% of kids who experienced childhood gender dysphoria and who are not affirmed or socially transitioned, eventually outgrow it."
The Sources:
Dr. Littman published a study on gender dysphoria, but it was removed because "the conclusions of the study could be used to discredit efforts to support transgender youth" (social pressure not to publish science that goes against the hegemony). It was later republished after careful scrutiny; the results were confirmed.
Irreversible Damage by Abigail Shrier was written using Dr. Littman's research as a starting point, conducting over 200 interviews. In one interview, a school representative told her "The role of schools has changed. Technically we are an educational institution, right? Reading, writing, and arithmetic--technically that's what we are. But schools have expanded to be the hub of a lot more social services. … Looking at schools as a hub for social justice, …" "Not that we're replacing family. But things that used to be the exclusive domain of family or society, we're asking schools to look at those a little more intentionally."
Even the first trans person I watched react to Shrier's research acknowledged that a prolonged medical evaluation was necessary prior to medical intervention.
Full disclosure: I've listened to the book. I haven't read Dr. Littman's studies or the other studies cited by Shrier. I haven't reviewed or even searched for credible studies with evidence for why Affirmative Care, etcetera is medically advisable (yet). This is one of many issues that I'm still looking into myself. But those are the counter-arguments.