What is on the pro side of teaching the subject a lot of parents don't want taught in school?

For example: Transexuality, the so called. “I don’t want my child to believe she is defective i.e I don’t want her to think I don’t like “female” activities so I must be a male, so I need to see the doctor kind of thing. The parents’ thinking: If it’s taught in school they will teach that it makes sense that a the child believes he/she is physically defective, so I don’t want her to be taught that.”