I'm explaining the term "social construct" as it relates to gender, that's all. Your perspective on the subject is absolutely valid, but also wasn't in question.
Societal norms are a powerful thing, albeit nebulous, often contradictory, and changeable (see: dress codes in a business setting). If you think societal norms don't carry authority, try getting a job with a face tattoo. If people trying to push their ideas on to other people disqualifies them as an authority, then there are no human authority figures.
Mack Beggs did not decide to be a girl again. He was required by the state of Texas to compete against girls because he was born with female genitalia. He continually pushed to be able to compete on the boys' team, but was denied. He'll finally be competing in the men's division in college.
This situation shows not that "transgenderism" is hurting people, but that the lack of acceptance of trans people hurts people. If he'd been allowed to compete where he belonged, no girls would feel cheated out of a shot at the state title. Their disappointment is not caused by the fact that Mack is trans; it's caused by Texas' failure to recognize it.