Watashi no Doukyonin, Kiss-suru to Okkiku Narimasu.

Nagisa is a new teacher who got tangled up with a delinquent, when a boy passing by suddenly kissed her on the lips...! Then, the boy turned into a grown man! And it turned out the man was her childhood friend, Yuto! Nagisa took him home to find out what's going on, but Yuto had already reverted back to his childish form.... "I'd like it if you kissed me once in a while so that I can become an man again." It's the start of a mysterious life together with a friend who doesn't know whether he's a boy or a man! (Source: CocoCheek, translated)