While your idea has some merit, there is a distinction to be made between writing a book about completely fictional scenarios involving sex with minors, and advocating sex with minors (telling people that it is fine in a piece of advocacy like on a blog).
People sometimes get these two confused, but they're not the same thing.
Someone can easily construct a fantasy world where doing that might be fine / unharmful (or in scenarios where it is harmful, it is clear to the reader that it is bad to act on), yet believe it is harmful in reality.
Between the covers of a book is another universe. In a way, imagine someone is transported to another planet which resembles your own.
There is also indeed the distinction between advocating for something and doing it. To pick a less charged example, I advocate for the legalization of most drugs (although, it doesn't mean I would support irresponsible use, much like how I wouldn't support drink driving). This doesn't necessarily mean I am secretly consuming cocaine, heroin, LSD, and all manner of other illicit drugs.
Another thing is that there is a difference between someone giving their opinions on the matter, and actively inciting someone to do it. If someone specifically instructs the reader to go out and do it, that is clearly very bad. But, I've never heard of such a thing happening, and if it did, it is more likely to be an internet troll. It is too damaging of an act.
I am of the opinion that text + anything which is digitally created is fine. Nothing which goes back to a specific abuse, although there are occasions where the victim of child abuse will chronicle their abuse in text, and I think they should be free to do so. There is also the case in Canada where someone did a retelling of an old story (IIRC Hansel and Gretel) which was more faithful to the original.
In practice, I imagine it would be quite ruinous to publish books which cover such themes under your own name, and it would be advised to use some sort of pen name.