It's Man's Nature To Eat What's Served

Sera, a sexually curious salaryman, has newly set his sights on Ryo, a new employee at his favourite bar.
He tries to talk to him about sex, a new kind of pleasure, but Ryo is popular with men and women of all ages and has a lot of experience.
Because Sera keeps pestering him, Ryo gets mad and teaches him about the pleasures of the backside...

(Source: Core Magazine, translated)

Note: Includes 1 extra chapter.