Kanojo no Ai ga Omo Sugite Tsurai

Mitsuya Haruyoshi is a devout Christian High-school boy since his father became a priest after his mother cheated on his father. He tries to avoid lust and desire because his father wants him to become a priest.

One day his female childhood friend Sonoda Hanako, the daughter of a sweetshop owner who goes to the same school invites him home.

Turns out her family shifted business to a sex shop to counter the effects of declining birthrates on their business. She reminds him that he promised to marry her when they were children and even gave her a ring made from Aluminum foil. She also notes that Christians only do lewd stuff with people they are married to, so if she does lewd stuff to him he'll have to marry her.