Mizushima was forced to go to a S&M brothel with his coworkers where he met Kazuma who works there. Mizushima broke up with his girlfriend who was into S&M sex but he was in denial of being a M until he met Kazuma, who is a S.
Vol. 4 Ch. 5.5 Face fetish x masochist
Ch. 17 He unlocked his masochistic side after his friend hit him with a backscratcher that is said to bring good luck. Lol.
Ch. 18 end {what's with this vague ass ending} Physically a virgin but mentally a whore— said by someone in the comments
Skimmed chapter 3,4,5,6. Rape, rape, rape, and rape.
Kudou Sensei no Majestic Days