Suezen Kurau wa Otoko no Tachi

Sera, a sexually curious salaryman, has recently set his sights on Ryo, a new employee at his favourite bar. He tries to learn about sex techniques and new kinds of pleasure from Ryo, who's popular with men and women of all ages and has a lot of experience. But Ryo doesn't take the bait, and when he pesters him, Ryo gets mad and teaches him about the pleasures of the backside...

(Source: Core Magazine, translated)