Otona No Koi No Hajimekata Manga

オトナの恋のはじめかた ; 成人恋爱初体验 ; How to make love in the office

From Starry Sky Scans: “What an erotic face, don’t be so greedy.” Shinoki has ended up working in the same company with his long-time rival from high school, Nagase. As usual, Nagase manages to catch everyone’s attention, irritating the hell out of Shinoki. At a drinking get-together with their colleagues, Shinoki ends up wasted and is sent off to a hotel where he spends one night with Nagase and gets fucked?!