Our First ~From Anal Masturbation To Friend Sex!~ (ぼくたちのはじめて)

Kaede picks up a book on his way home from school, and it contains otoko no ko having anal sex.
Wondering if so much pleasure can be had from the anus, he tries anal masturbation himself, but he gets caught in the process by his friend Iori.
And from that, they end up trying out anal sex together!
The relationship of 5 boys changes as they become captive to anal sex.