Kono Koi wa Kamisama datte Shiranai

On Christmas Eve, Mao is kicked out by his live-in girlfriend. Which happens to also be his birthday. This leaves Mao feeling abandoned by the world. While wandering aimlessly through the city, he collides with a man: Mikoto, his first love. Even though Mao is rejoicing over their miraculous reunion after twelve years, Mikoto doesn't even remember him... But when Mao tells him that he has nowhere to go, Mikoto invites Mao to his home. Mao follows him back home, except Mikoto is suddenly doing lewd things to him...?!

(Source: Kadokawa, translated)