Wow! Want to say thank you for all the interesting, well written opinions/ explanations to this difficult topic.
To be honest, it actually helped me. I feel more safe now with my approach.
I'm writing a mature story, setting is dark fantasy. I don't write a lot of gore/ horror elements, but most of the other topics, well... 
I marked the story as mature/ 18+, even checking the boxes carefully for the specific topics on every chapter, always fearing I could miss something, so someone gets offended/ hurt.
On the other side I always had this feeling, that it's sufficient that I wrote in the description what the story contains. And that it's the readers responsibility to know if they're okay with these topics.
I was often a bit puzzled by the fact, that some stories that you can buy easily (quite possibly even as a minor) in book form in every store around here (not even tagged 18+), would be a "no go" online without big warnings. (Yes, most books got through some kind of testing, and online stories mostly don't. But that's why I talk about a series with lots of "vividly" described violence and sex.)
So, well... Where lies the responsibility in this case? It's the reader's responsibility. And with all I've read from you, I feel more on the safe side with this perspective now. 
Thank you!
Some thoughts I have to the general topic in addition:
I'm definitely for showing different perspectives, so the reader can get a feeling for what is "normal".
Example: Some genres (like fantasy/ sci-fi) give the opportunity to dive into worlds, where m*rder, r*pe (we do censor these words?) or other taboos are something common. Something that isn't that horrible/ shaking like it is in our reality (at least parts of it, quite depending on the society you're living in, I assume).
If so, this difference should be explained through/ in the story; at least that's my opinion. So that it can't be considered normal for everybody, just for this specific reality.
And if it's not normal, I'm totally with @TheKillingAlice, it should definitely be made clear that something's off.
I, personally, am trying to draw a line between "normal" and "off" when it comes to hurting someone (physically and mentally). But this is super difficult, I know that. And it says a lot about my own boundaries where I draw this line, and not much about a greater "good or evil". I think, that goes for everyone. My opinion is that this means, I have to be open for criticism and discussions. (But I don't have to put up with inane slurs/ insults.)
(For me, for example, everything around LGBTIQ+ is fine. But I know that others, for whatever reasons, go crazy about this topic, feeling attacked from the mere mention. So they would claim they get hurt. I can't see a reason for that, so my line is at another place. Am I right? Am I wrong? Who knows. I think I have valid reasons for my position, but the others will claim that too.^^ Still, "I just don't like it" doesn't count in my eyes.)
Btw, I would love to have a tag for "please be quite a bit older than 18 to read this story", cause I sometimes get the feeling that 18 is just not old enough to be the right audience for my story.
But I know that's kind of silly. In the end, age doesn't say that much about how resilient/ understanding someone is.
Oh my, I think I've digressed quite a bit. Sorry for that!
I felt the urge to write something to this topic, but it's late, I'm tired and I'm not sure if I made any sense. 
Still, thank you all again for your insights! It was really helpful.