I did get that, but I don´t understand your point of the dark end. Do you mean the manga with that or the doujin ??
When we have to talk about morals in this manga/doujjin then there is a lot to discuss xD
Yes in the doujin they are panels how she fucks with the teacher, but there is nothing with a threesome between agawa, teacher and Gotou. It happens during the time where both of them don´t have any contact besides during school.