I never fap to hentai, but pretending that I would, this came to my mind:
My slave by Distance -> too sad : the story is about a boy who inherits his fathers slave, a beautifull girl. But she is not the ordinary slave for life, the "master" pays a fee and gets to own her for a special amount of time, in this case half a year.
In the end they both fall in love with each other but after his time with her runs out she must move on to the next job and decides to not give in to her feelings, thus leaving the boy devasted.
Castrato by Kokurio -> too fucked up : up till this story I never read any tags or titles and only paid attention to the art.
Big Mistake! The story is about a boy who wants to sing in the church choir and asks a nun about it. She asks him if he knows what this means and he says yes. Turns out that you must be castrated in order to join the choir. She lets him have sex with her, cuts of his dick, makes a strap-on out of it and fucks him afterwards. At the end of the story she looks for the next victim. Till today I ask myself why I read this story to the end, it's one of those you want to erase from your mind...
Also not fapable:
- ball kicking/crushing stories
- cuckolding stories
- guro, scat stories