Oppai Heart Kanojo wa Kedamono Hatsujouki - Season 1

A tranquil school existence shared by Ryuya and his friends, including his older sister Miya, is disrupted following the accidental consumption of a medicinal beverage. This substance induces a state of heightened sexual sensitivity and arousal. Ingesting it causes the female students' breasts to enlarge, creating a physiological need for intercourse to achieve relief. Furthermore, the development of a curative vaccine necessitates pregnancy. In response to these conditions, Ryuya engages in sexual relations with the affected girls to alleviate their symptoms and facilitate the vaccine's production.