This is actually part of a series of oneshots called "Lips That Don't Resemble His" aka "Anohito to Nitenai Kuchibiru". However, most of them are quite average and uninteresting.

A former gangster has turned into a regular office worker, but he still sleeps with his gangster lover. But one of the newbies doesn't like the fact that his favorite 'boss' is having a relationship with an office worker.