How do I help my teen daughter understand why porn is bad?

[Gladius]

Einstein once said* "If you can't explain something to a child, you don't really understand it yourself."

*Maybe. People say he said it at least.

Anyway, my point is, if you can't explain it, maybe you don't know the reason yourself. Or maybe... there isn't a reason and unlike your daughter you just accepted it unquestioningly when people told you.