Sometimes, I enjoy reading about abusive relationships in fiction. This isn’t an endorsement of abuse in real life. I’m open to talking about the toxic nature of this fictional relationships (or I think I am). Yet I don’t slap on a ‘yes, I know this relationship is messed up’ disclaimer every time I mention them.
Purging fiction of all toxic behavior is ridiculous. Many people experience abuse, and they deserve to see themselves in fiction. They need to know they’re not alone.
Some time ago, I read an essay by a bookseller who feels uncomfortable whenever 12-year-olds buy Colleen Hoover books. (I haven’t read anything by CoHo). This bookseller read one of her novels and felt that it romanticized relationship abuse. She doesn’t want 12-year-olds to think that’s acceptable behavior. Yes, she sells them the books anyway upon request.
Should writers always make it clear that they disapprove of abusive behavior? No. That becomes high-handed moralizing, which makes readers roll their eyes and, ultimately, backfires. And yet, without that, some readers may believe that writers approve of their characters’ destructive behavior.
Our culture in the 21st century regards murdering 6-year-olds as wrong. When a story features a murder of a six-year-old, even if the writer doesn’t outright say ‘this is wrong’, the assumption is that the writer condemns child murder.
However, our 21st century society accepts many forms of relationship abuse. Depending on the issue, I can’t assume that the writer recognizes that the toxic actions they are romanticizing is wrong. Even more disturbing that what the writer thinks is what readers think. That bookseller didn’t seem to care much about what Colleen Hoover believes. She cares about what the readers will believe.
Also… many readers enjoy reading about abusive relationship dynamics. Like me. They generate conflict and attract my eyeballs like a train-wreck.
I tried to come up with an answer for what fiction writers should do, and as soon as I wrote something down, I came up with a rebuttal. So instead, I’ll say: we all need to be better at taking care of each other in real life. That’s more important than what happens in fiction.
The Notes Which Do Not Fit 
And then there’s reading fiction produced in the past, or in very different cultures, where neither the authors nor the readers could be expected to share any of our values . . .
As you suggest: without conflict of some sort, is there even a story? and telling the reader what to think and feel is merely preaching. “Reading” is not a single activity, but many possible activities, undertaken for a dizzying variety of reasons. “Pleasure” and “enjoyment” are also various. There’s the pleasure that comes from recognition of what’s happening in the text, and the pleasure of being able to see “behind the curtain” and recognize the craft deployed in the text. Then there are more . . . let’s say “basic” pleasures. I remember overhearing a conversation from the next room (while silently reading! and so my presence wasn’t known) between my parents. Dad, a high school teacher, was reporting what the school librarian had told him: one of my classmates, the librarian feared, seemed to be reading romance novels “for masturbation.” “As long as she’s not doing it in the library, and the books come back in good condition, what’s the harm?” was Dad’s thought.
My academic training was in literature, which I mention to suggest that the kind of reader I imagine, by default, is much like me: interested in how human relationships play out in the text. Who has power? Who abuses it? Who is mindful, who is unawake? What do they want, and why? It sounds like you and I have similar default imaginary readers. But other readers come to the text for their own reasons: to escape for an hour, to acquire cultural capital, to get a grade in a class, to learn the law or vocabulary. Or even, like my classmate, to get a thrill. We have to hope that the right text strikes the person who needs it the right way, when they need it. Sometimes it does!