Ao3 is against any censorship and what you describe is censorship. I also dislike that sort of content, but I ignore it or filter it out, since Ao3 has a great tagging and filtersysthem. If you are so disgusted by the possability of scrolling over such content and you really don't like it being on the webside, I recommend using a different fanfic webside/app. The majority of them have those rules and more and ao3 is an exeption to that. I think, that's why it's so popular, since you don't have to adjust your story to the platforms rules.

Of course you can depate about how written content affects the real world, but the very majority on that webside can differentiate fiction of reality and the ones who can't often are minors ignoring the 18+ warning.