Boku wa Aniyome-san no Karada ga Ki ni Narimasu...

Ayumu is a timid boy who refuses to go to school, and his family is constantly scolding him for being a shut-in. His brother's wife, Ms. Midori, is the only one who treats him with kindness and sympathy. However, Ayumu's affection towards her becomes an obsession with her voluptuous body, and secret masturbation sessions eventually lead him to cross the line...
(Source: Irodori Comics)