I write fanfiction and I have done so for twenty years. I am quite comfortable with the moral ground my own stories cover but I suspect the Purity Police might see things differently were they to read my stories. Off the top of my head, I can think of several areas they might find problematic in my own fiction.

I have teenagers having sex in my stories, because teenagers do. Teenagers without hormones? This would sorta be unrealistic.

I have a pedophile in some of stories ... a bad guy ...  who eventually gets his head lopped off by a very pissed off Immortal. Before he does, however, there's a scene with a fourteen year old girl that also involves quite a bit of alcohol. In a certain light, this could be seen as gratuitous, but the scene is there for a reason: I wanted to show one way that pedophiles operate, and I wanted to show what was going through the girl's head as well.

Truthfully, I have multiple sex scenes in various stories -- I don't write extremely explicit sex(*) but sometimes specific details are necessary for forwarding the plot so I don't necessarily fade to black.

In my fic  I have one male/male relationship with one of the characters being slightly underage by our (US) laws because he is seventeen. I have one story that has a humorous but somewhat graphic opening involving coitus interruptus between a human and a demon who are engaged to be married. I also once wrote a joke fic that involved e-mails between most of the characters of Disney's Gargoyles -- and let's just say the innuendo was rampant and silly and multi-layered and the fic  was very well received. And that's just a quick list off the top of my head.

I maintain all of my stories have artistic merit -- generally, I have a point for what I write (though the joke fic was purely for entertainment value!) -- but the Purity Police could well see things differently.
It would be very easy for the Purity Police to misrepresent the content of my stories and twist my intent around to look like something nasty and evil.

The link above for Pornish-Pixies is about a fanfic community that was deleted because of incest fics. While incest fanfic isn't my cup of tea, stories about incest can have artistic merit.

I can think of one or two Supernatural Wincest stories which have been recced to me and which deal with the topic in an intelligent manner. I am not going to link to it, but one I am specifically thinking of covers other character's reactions to Wincest.  It was a very thought provoking story. If both adults are adults and consenting, does society have any business dictating what they do in the privacy of their bedrooms?

I'm not going to answer that question right now (that's a blog for another day) but the story argued both sides of the coin and quite successfully, too.

Pornish-Pixies was deleted for incest, but it seems some other communities have been targeted for sexual content involving underage characters. Not, mind, real people, but fictional characters. While I am one of the first people to stand up and protest stories advocating pedophilia or which portray sexual activity between very-much-older characters and minors (chan), some of the stories on these communities had to do with teenagers and didn't deserve deletion.

There's a world of difference between a six year old and a sixteen year old. I suspect that true kiddy porn involving stories about small children would have been met with open hostility on most of the communities targeted. I was not a member of any of the communities and so I can't say for sure, but I know fandom and fanfic writers in general well enough to predict the response. (There may have been some specific exceptions judging by the names of the communities but they'd be the exception, not the rule.)