Houtou Musuko To Koi No Ana summary:

Sousuke's lust for women is too much, so he's transferred to a high school in the country. While staying with his aunt, he encounters his first love, his cousin Minori-chan. However, it's his first time seeing them in 11 years, and come to find out, his beloved Minori-chan is a man...and a teacher at his school! His world comes crashing down, but just when he's about to give up, he witnesses Minori doing...lewd things to himself?!