Sono Hada no Atsusa wo Oboeteru

"What would you do if I told you your partner that day was me?"

Gen, a hairdresser, has long been in love with his childhood friend, Koutarou, who likes Gen's older brother, Kanade. One night, Gen spends the night at a hotel with a drunken Koutarou, bolstered by the thought that he might have a chance. However, Koutarou mistakenly thinks he slept with his older brother, Kanade. Gen's used to being mistaken as Kanade, but he's hurt when he finds out that he was mistaken for Kanade even on the night they felt each other's heat, and tries to distance himself from Koutarou...

(Source: Hakusensha, translated)