Don't Be Cruel: Akira Takanashi's Story summary:

Polite and proper college student Akira Takanashi has a secret. Back in high school, he fell deeply in love with his teacher but was spurned. Now, whenever the pain of his heartbreak gets to be too much, he finds comfort in the arms of med student and notorious playboy Shimakawa—very adult comfort. Shimakawa would gladly go out with him, but Akira’s past heartbreak means he insists on keeping things strictly physical—well, until his obsessed younger brother Jutta arrives to disrupt this comfortable routine!