Sugar to Mustard

It's a world where S and M matches get systematically created. Office worker Masachika (M) is introduced to the younger university student, Seiji (S). Masachika intended on taking the lead, but he was led around by Seiji's unexpected S techniques, and made to come relentlessly! With their physical compatibility being perfect, and their natural transition into living together, Masachika plans to coach Seiji into becoming his type.

However, the gap between Seiji's kindness and his relentless attacks makes Masachika's heart skip a beat?!

(Source: Frontier Works, translated)