See this is the kind of reaction that makes those shirts so cool to a fifteen year old. It drives people to the extreme, kids feed off that, and use those kinds of behaviors to equalize the power imbalences inherant in the adult child dynamic. Your outrage is the infastruture for their counter-culture.
Think about it man: A MAGA hat can drive a fully grown adult male to take drink and splash it in the face of a 17 year old, and a NRA t-shirt turns a teachers head out to the point that ity becomes a news item. Thats alot of power to give to a child. If you can dress up as chick and use the chicks bathroom and everyone is compelled to suspend their belief suystem and subvert thier world view to play along, then whats wrong with a friggin t-shirt?
You can't make it this easy for them.