I'm not necessarily disagreeing with you, but it's interesting how manga/anime is invoked in these discussions, often unintentionally or even unknowingly recalling the image of 'weird Japan', a supposed hotbed of child abuse and perversion, where no material, much less anime and manga are safe for Western eyes, supposedly more moral and discerning.
I recently read a paper about this kind of discourse, in which the authors pointed out, quite rightly in my judgement, that the popularity of such readings in the mainstream Western press projects a certain idea of fiction and its relationship with reality which is not shared by many in Japan, and certainly not by the consumers of this 'pornography and perversion' according to ethnographic research on their opinions and perceptions.
Second is the idea of 'stumbling' upon porn. Some people even class Neon Genesis Evangelion as 'perversion' in its own right, and recently in the US, portrayals of adolescent sexuality in stories and books, which some lawmakers are trying to remove from libraries. I'm of the opinion that the sparation isn't nearly that easy. Teenagers writing Harry Potter slash fanfiction is a far cry away from Blender animated monster porn, and a fannish teenager or younger will have a greater interest in the former.
Consider just how well moderated most Internet communities are. The chance of seeing porn (as opposed to the anime equivalent of a very softcore porn gif) on r/all, or even r/anime is slim. A child is much more likely to 'stumble' accross it if a friend they know in real life sent it to them through Discord. The only other place I can really think of a child 'stumbling' in a zone meant for children would be Twitter, but then we're talking about Twitter, not 'reddit', 'anime', or 'manga'. In other words, I'm saying that just because the material exists, it doesn't mean it's innocently 'stumbled' upon by a wide-eyed child. Children get curious, especially when you tell them not to do something, or society provides that narrative.