Please help me!

I am looking for a manga where a boy is excited to live in the dorms of his school because he's an only child and looking Forward to having someone to look up to as a older sibling. Then he is sad to find out that his roommate is mean and not at all brotherly. The only way for him to be nice to him is to have sex. Even though he was kind of joking the boy took it seriously and decided to practices with a cucumber. Does anyone know what yaoi manga this is? Please I need to know its killing me that I can't remember it!!!!!