Himitsu wa Sarashite Mederu Beki

Tokumoto is admired at work for his serious and reliable nature, but he has a secret. He likes to play with himself using adult toys. In his late twenties, still a virgin, with a well-trained body but no practical experience, the idea of finding a boyfriend feels like too much trouble. That is, until the friendly and outgoing younger coworker Kayama catches him shopping for toys. Before Tokumoto can recover from the embarrassment, Kayama suddenly asks him, "Can’t I be your partner?"

(Source: Kaiohsha, translated)

Note: Includes one extra chapter.