You don't think adults should be able to choose what they do with their bodies? Does that extend to drug use?
It depends upon what it is they want to do. I don't use drugs anymore but if adults want to take drugs that is their personal choice.
There needs to be many more legal hurdles people seeking or who want a sex change operation go through:
1. Many hours of counseling or therapy, and not only by gender/trans therapists, at trans clinics, etc. No surgery or hormones given to anyone who is a teen or child unless it is actually completely medically necessary.
2. Years or decades of living as the sex they are genetically, biological, and will always be, go through puberty, have sex as a mature adult with other adults, etc.
3. They watch many hours of videos of sex change operations, get told of the many dangerous and negative aspects of the surgery and dangers of taking hormomes and are told that taking hormones and having surgery will permanently change their body, can make life and sex worse, and that the hormomes and sex change surgeries are not going to magically improve or somehow get rid of all of their mental health issues, personal life issues, etc. and that they will always be the sex they were born, the sex change surgery does not actually really give them the genitals or body of the opposite sex, etc.
3.That if they do decide to transition that people using their former name, and people using the wrong terms or words to describe them are going to happen, that it is not traumatic, and how people change with time and education.