Seifuku, Shinya 2-ji.

Overview

Eisei hasn't been close to his neighbour and childhood friend, Tsukiou, since he turned down Tsukiou's confession when they were younger. After his ex-girlfriend publicly revealed that he was bad at sex, Eisei stopped going to school. One night while feeling alone and depressed, he sees Tsukiou again, kissing an older woman in front of his house. Saying that he'll stay with him until he's ready to go back to school, Eisei spends his time in Tsukiou's room as if his confession never happened and his wounded heart gradually starts to heal... That is, until Tsukiou gets a text from an older woman; even though he had told Eisei he loved him, he put this unknown woman first. "If you don't want me to hate you, teach me techniques to have better sex." Tsukiou's gentle demeanor completely changes, and suddenly Eisei is completely at the mercy of Tsukiou's cruel-yet-incredible sex...? No longer a child, not yet an adult. (Source: Takeshobo, translated)

Source: AniList