Kyouya and Suzu were brothers. Despite being bound by blood, they desired each other.

Kyouya made love to Suzu and vice versa. He raped him, and got raped by him.
They knew it wasn't normal. They were disgusted in themselves, and yet, Kyouya couldn't find it in himself to reject Suzu.

It was his way of atonement.

[Translated from DLsite]