I edited the original comment to note that hentai manga (not so much video, though, for some very odd reason) is often of high plot quality, I would guess because in manga the scriptwriter often also is the one who draws it, and when you're putting in all that work to draw something line by line, you won't be able to avoid wanting to make it beautiful and wanting to add plot.

Then I noticed that people were no longer voting up the comment, and edited back out - I would guess that mentioning manga made it seem lower-prestige.

But (he quibbled) I'm not sure I'd classify F/SN as erotica in the first place, even if it's technically in the erogame genre; it's a novel with some sex scenes.