Strum My Voiceless

The "Secrataries' Association"; a secret underworld gambling den where business executives use pawns from the lowest caste of society, referred to as "secretaries," to engage in sex battles within a fighting arena. The outcome of the battles care determined by who can ring the other man dry.

Ryuga is the undefeated No. 1 ranked "Secretary" in the Official Sex world. Having just won his recent battle and heading home, winnings in hand and smoking a cigarette, he bumps into a well-dressed man, burning the man's suit with his cigarette. An argument ensues, and Ryuga ends up giving the main all of his winnings for the damage to his suit. The encounter leaves him never wanting to see that man again. But the next day, Ryuga heads to his new employer's company, only to find that same man there... and he's his new boss, Onizuka! Ryuga, whose battle abilities are doubted by the sadistic and highly capable executive, is provoked into trying to prove himself physically, but... he is instead overpowered by Onizuka—?!

(Source: Takeshobo, translated)