Yoake wo Matsu Kimi no Tame ni

Nao, a 40-year-old man, goes into a gay club with his close buddies and sees a handsome, young man in his twenties, bewildered inside said club. He finally meets Touma after approaching him in order to help him since it was his first time, and after having a brief conversation and leaving the club, Touma and Nao end up having sex together. Nao realizes how compatible they were and was doubting whether he should call Touma for another meet-up again, until Rei, Nao's son, visits him in his work, together with another a boy he is friends with in school, which happens to be... Touma.

(Source: MU, edited)