Not sure how I feel about the smutty fanfiction, it was a pretty normal part of my teenage years to read increasingly edgy fanfiction. I do find it odd if adults are reading smut about underage teenagers though.
I'm in the same boat -- I'm not very keen on censorship or shaming writers, mostly because I write fanfic of my own (not for Harry Potter) and online communities I've belonged to have been attacked by fandom "antis" (those who think certain topics should always be off-limits, even if the characters involved are adults). I have / have had some niche reading and writing tastes that I have been judged and felt shame for.
On the other hand, I too raise a bit of an eyebrow at adults reading underage smut.