It is the fanfic author’s responsibility to protect children who aren’t old enough to read tags and warnings to just not write anything with adult themes ever lest a child be exposed to adult content on the internet.
When Mark Twain made his statement, “censorship is telling a man he can’t have a steak just because a baby can’t chew it”, I’m sure he never imagined that one day it’s going to be the baby making that demand, personally, on twitter, calling for this indigestible awfulness to be completely removed from civilised society. Also the baby is 23 and also wants literal, actual beef steak banned, too.