Dr. M no Sasayaka naru Yokubou

I can't believe what I'm feeling it when he's being so gentle with me.... I'm a terrible sub! One day, Dr. Shizuka, a masochistic dentist, falls in love at first sight with Ontake-san, a young leader of the Onitora-gumi. He thought Ontake-san would the ideal dom, with his low voice and a steely gaze. But he is way too nice, even though he is a yakuza gang member! After they become lovers, Dr. Shizuka planned to train Mr. Ontake to become a great dom, but...?

(Source: Gush Comics, translated)