I’m not inventing grey areas: I’m pointing out that they already exist. You’re insisting that it’s impossible for anyone to argue about whether a given work does or doesn’t fall within the parameters you’re setting, because you’re pretending the only salient examples are the most egregious ones, but as I keep saying, they’re not. Once you set out to ban a particular type of content, there will always be borderline cases, and if you can’t figure out a coherent means of dealing with that, then you’re not going to do any good.
Vladimir Nabokov’s Lolita, Stephen King’s It, V.C. Andrews’s Flowers in the Attic, Alice Walker’s The Colour Purple, George R.R. Martin’s Game of Thrones – those are all published novels that feature graphic sex either with or between underage characters, nor are they anywhere near being the only such narratives that are legally available. You keep insisting that this is only about badwrong porn (as though abuse victims with intrusive symptoms that tie their arousal to their assault don’t exist – which, spoiler alert, they do), as though there’s no overlap between “sex scenes that aren’t written as porn” and “sex scenes people treat as porn anyway”.
I’m not arguing that fiction has no impact on the reader: I’m saying it clearly does, to both positive and negative effect, which is why tags exist to help readers navigate difficult content. Beyond that, the onus is on the individual to curate their experience.