in my humble opinion, we already have “thing that can be done to prevent that” and that thing is tagging system. people can filter out things they don’t want to see. anything else other than filtering out tags about things you don’t want to see would be “preventing other people from having access to things you don’t want them to see” and that is censorship.
I know you already mentioned a domino effect censoring one thing can lead to, but I’d like to go into deeper details about this anyway, since there are still some people out there who could use this as a reminder.
no censorship means no censorship. no censorship does not mean “only things I personally am comfortable with are allowed, but there’re still limits for things I’m personally uncomfortable with” — because that is censorship, and like you said, it is a slippery slope, it causes a domino effect. you cannot censor something and allow only the things you’re personally comfortable with to have a platform. once you allowed one thing to be censored because you deem it Too Much, next thing you know, everything that isn’t vanilla and heterosexual will be censored because someone somewhere also deems it Too Much.
so the point of anti censorship stance is that it protects everything, things people aren’t comfortable with, things that are taboo, disgusting, morally wrong, socially unacceptable, you name it.
people wouldn’t try to censor something that they have no problem with. no one tried to censor fluff because fluff is Good. no one has a problem with fluff. censorship only comes for things about which some people have problems. so yes, things like the stuff you mentioned. but also somewhere, something like being queer or trans is deemed unacceptable by some people (to clarify, I am not saying that being queer or trans is wrong, I am saying that some people deem being queer or trans unacceptable and wrong). so once “this thing can be censored because this person deems it unacceptable and wrong” happens, next thing you know, you’ll be dealing with “that thing can be censored too because that person deems it unacceptable and wrong”
writing non-con fic? banned. writing minor x adult fic? banned. writing whump fic? banned. writing fic that includes violence? banned. writing gay romance? banned. writing about a trans character? banned. because all of these topics have already been censored somewhere because “there are people who find these topics wrong and unacceptable”
it’s either everything (that isn’t fluff and vanilla and heterosexual) is censored, or nothing is censored.
ao3 was created with the main purpose of housing things that weren’t allowed on other platforms, as well as things that were being censored by some media somewhere. so yes, ao3 was created with the main purpose that these things are rightfully allowed to exist, anyone who want to access these things can freely have access to these things. and “these things” also include things that are macabre and taboo, because these macabre and taboo things cannot harm anyone in real life, and whether or not you like it, they count as a form of art too (yes, art can be macabre and taboo, art can be outright socially unacceptable, morally wrong and extremely disturbing).
ao3 was created with the main purpose that there would be no censorship. ever.
so I would like to answer your question, “why are we making these fics accessible for anybody without any sort of prevention?” with this question “do you believe people should be allowed to access video games and movies that depict violence and/or other bad things without any sort of prevention?”
the answer, for me, is yes, these things (dark fanfiction, video games and movies that depict violence and other bad things) deserve to have a platform and be accessible to anyone who wants to access them, because the point of it is that they are fiction. they are not real.
people who like playing video games that depict violence aren’t necessarily a violent person in real life. they don’t have to condone violence in real life. they just like violence in video games.
people who like horror movies or movies that depict violence and/or other bad things don’t condone violence or other bad things in real life. they just like consuming bad things in a controlled and fictional environment, aka movies.
the same logic applies to people who enjoy dark fanfiction.
these are all forms of art. people who enjoy macabre art, for whatever reason, aren’t going to hurt people in real life because they enjoy macabre art.
a person’s moral compass and never be judged by the fictional things they consume and/or create.
more often than not, these dark fiction, fanfics about the things you mentioned, are created and consumed by the kindest, sweetest people you will ever meet.
I love writing and reading explicit non-con fics. I have been fantasizing about non-con scenarios ever since I was very young (probably when I around 7 years old and didn’t know what whump was). non-con was one of my first whump awakening experiences even back when I didn’t know what whump was. that does not mean seven-year-old me was a predator. that does not mean adult me ever condones this thing in real life. it just means I find joy and comfort in dark fiction, keyword: fiction.
people who hurt other people in real life don’t do what they do solely because they read fanfiction or because they like playing video games or watching movies that depict violence and/or other bad things. people who hurt other people in real life do what they do because they are bad people, possibly because of something they may have gone through in real life (not because of the fictional things they may or may not have consumed), and, without someone stopping them, chances are that they will do these bad things anyway, with or without dark art.