So, let's dive right in!
I'm sure I've mentioned it once or twice before, but I do not believe that any subject matter should be off limits when it comes to storytelling media. Be it sex, violence, drugs, or adult themes across the board, any topic has the potential to be used in an appropriate, artistic, informative, or just plain entertaining fashion. However, I do believe that all of these topics have the potential to be used in inappropriate, damaging, and irresponsible ways.
In my view, an author has a degree of responsibility towards their readers. This will vary depending on the genre and subject matter they're dealing with, but when a person picks up a book (or engages with any other form of media) they're entering a mutual understanding with the creator that sets up various expectations in their mind. If a story is light and fluffy and fun, the reader is unlikely to take anything that happens too seriously. If it's a quick and smutty erotic romp, they're going to be expecting kinkiness and sizzling sex across the board from start to finish. But if the story is serious, realistic, and poignant, you can bet your money that the reader will be listening intently to a lot of things the writer has to say.
This is a shades of grey topic, and the degree to which this investment by the reader can vary is enormous, but in its most simplistic form this basically means that if an author sets themselves up as a figure of authority, chances are a few people are going to take what they say to heart.
Now, I'm not saying that every reader will necessarily act on what their favourite author conveys to them through their books, but I couldn't sit here with a straight face and pretend that fiction never affects us on a personal level in some way. We've all laughed, cried, and re-examined various areas of our lives because of the stories we've been told. Stories are one of our oldest and most effective learning tools. No author can be completely absolved of responsibility when it comes to what they write under the excuse that "it's just fiction". Fiction can often be just as powerful as a classroom when it comes to putting new ideas in our heads.
Phew, looking back at the paragraph it seems like I'm making stories sound a lot scarier than they are. So what's my point in all of this? Should we never talk about dark or disturbing subject matter for fear of it having a negative effect on our readers?
Of course not. That would be absurd. Let's not forget that learning goes two ways -- it can both encourage and dissuade us from new ideas. This is why, at least in my view, the most appropriate uses of dark and unpleasant subject matter in fiction are either those that present it in such a fantastical or absurd light that it becomes detached from reality (like say, the ridiculously over the top violence in a Quentin Tarantino movie, or a forceful bodice-ripping sex scene fuelled by pure erotic fantasy), or when it takes the material 100% seriously and uses it to tell a cautionary tale.
Good examples of this latter point would be movies like Saving Private Ryan, or books like The Clan of the Cave Bear. Both of these stories deal with horrific violence and incredibly dark themes, yet the thematic purpose of this material is to unsettle and disturb the audience. Quite the opposite of encouraging negative behaviour, these stories make a clear point of illustrating how abhorrent acts of violence are in a way that resonates with viewers/readers on an emotional level.
The flip side to this is essentially propaganda media. Thankfully, not many storytellers in the grand scheme of things are setting out to manipulate their audience in a way that could be construed as morally dubious, but it's still an issue to watch out for. My overall point in all of this is that authors have to be careful in how they present their subject matter. I believe that far more "unethical" writing is a product of negligence than it is of intent. When authors present serious subject matter in a semi-serious light, that tries to be realistic while failing to appreciate the tact required to handle serious topics effectively, then you end up with misinformed and misleading ideas creeping into your work. It's things like weak doormat characters in BDSM novels that never question or stand up to their partners that bug me. Romances that unintentionally glorify borderline-abusive relationships, or realistically presented power fantasies that hamfistedly construe unbridled violence as something to be applauded.
Fortunately, these ideas have pretty limited potential to affect their audience in and of themselves. But having said that, think about how our perceptions of things like weight and body image have been influenced by the media over the years. Not just in advertisements, but through TV, movies, and even books almost across the board. It's hard to argue that widespread portrayals of heroes and heroines as beautiful, slender, well-proportioned individuals haven't had an effect on many of us in one way or another.
Even simple ideas like how you choose to portray your characters can have an effect on your readers, and when these ideas start to self-perpetuate, they have the very real potential to alter how others think and behave.
It may be a small effect that our writing has, but it's not one to be ignored out of hand. At the very least, it's something every author should be aware of in the back of their mind. When dealing with ethically provocative material, make sure you know how, and more importantly why you're choosing to handle it.
An author shouldn't be crippled by the weight of responsibility behind what they write, but it should always be there in the back of their mind, asking them to question, think, and consider.