I assume it's less about story and characterization than there is that element of grooming minors, abuse (the dislocation of the shoulder would be considered a crime in almost any civilized nation). The sense that the sensei is abusive, even if it's a complex situation, to any outsider, he's making use of the weaknesses/trauma of a character and had done physical violence to him, and he's had sexual activity in violation of professional ethics with a minor (maybe many minors).
To some people, that is repulsive and will downgrade against other factors.
I dislike sensei, but I can see the interesting aspects of the story
Lots of folks in this story seem to need therapy, not sex.
I don’t know why this is rated 8,7 cause it deserves so much more. At first I was suuuper confused about Tengoku’s character but I like both him and Aoki. Cliffhanger is killing me :)