I think it goes both ways. A lot of fiction revolves around situations you don't want to find yourself in in real life. There's nothing wrong with that. Around the 8 minute mark in this video, there's a section on portrayal of potentially dangerous, upsetting or sensitive subjects which covers it really well.
Essentially, people have the right to produce whatever they like and it is up to the audience to curate their own experience. However I think that just because you can create whatever you like, doesn't mean you should be irresponsible about it. I take the stance fiction is not reality, but fiction can effect reality, and this is often the goal. You want to evoke emotions in your audience, most the time. So, if I then hurt the audience by including a triggering subject, I'd be pretty upset. Which is why I think, while it's up to the audience to curate their own experience, how they supposed to do that without some kind of trigger warning? How are we supposed to say "if rape triggers you stay away from stories with rape in", essentially "don't like, don't read", if stories don't tell you it's there until it happens, and by then it's too late?
As for where we draw the line, I think it's important to have a bit of historical context, for instance older books are infinitely more likely to be kinda racist. I think while we accept that's a product of their time, there should be some kind of warning still. Lolita, for example, should have a warning (as if most people don't know it by reputation) so you can decide if you want to face that in order to analyze the book or find another. I'm sure it's not so supremely perfect you can't find similar without the content, after all.
Essentially, both should be responsible. Basically, "don't like, don't read" and "dead dove, don't eat" need to be respected (apparently that can be a little less well known, essentially, if something is marked and you read anyway, you have no right to complain when you find the content marked upsetting) and needs to be respected.