OMOI TO JIKKAN TO SONO MUKOU

Omoi to Jikkan to Sono Mukou summary:

Ch3: Katsuragi, a boxing instructor, has a habit of falling asleep at a bar he frequents, oblivious to the fact that the son of the bar's owner, who also happens to be a student of his, is kissing him every day! Apart from this couple who're set apart by their age difference, you can also enjoy the passionate romance between Ichiha, who is a menacing boxer in the ring but becomes shy out of it, and Hironari, a fellow boxer who finds him endearing. A collection of love stories between ardent youth at the gym! (summary from Nakama)