[Chinpan-bu (Chinpan)] Ossan Kirai no Hitozuma Kyoushi ga Nikubenki ni Ochiru Hanashi | Story About an Old Man Hating Married Teacher Falling Into Being a Meat Toilet [English] {Doujins.com}
If I had a nickel for every "Haha Retardina-chan! I'm blackmailing your husband unless you become my sex slave for X period!" Story I've come across, I could retire very, VERY well.