The only instance it becomes harm is when rape, pedophilia, homiphobia/transphobia, zoophilia, incest and such topics are normalized, or more specifically romanticized.
By this i don't mean those things shouldn't be mentioned, because awareness reduces losses, but romanticizing it is entirely different.
I've seen many young children (10-14) think those are normal and okay because of certain fanfics, and I've also seen grown adults believing that too. Fanfiction is a free creative space, and while i may not like everything, I wont go yelling at others for it (for example,i dont like dark fics with rough or twisted themes, but most of the time it's not harming anyone, so why would i ruin someone's fun?)
Yes, a child's internet safety is the responsibility of a parent, but society isn't just children. There are victims and even criminals roaming freely on these apps, so while fanfiction is a space where anyone can do anything, we should at least draw the line where it begins to harm society. I agree with those above posts entirely, and im sure they meant this too, but i need those of you who didn't remember this fact to remember.
And ffs don't go sending people death threats even when you encounter fics like these. Everyone behind the screen is a human, so keep that in mind as well when interacting with others in a fandom.
