Shinnin Kyoushi ~Kateikyoushi wa Chitsunai Shasei ga Osuki?!

The game's protagonist is a home tutor. He harbors a secret crush on Kashiwagi Maki, one of his students.
Then one day, suddenly, there's a confession of love and their sex life begins.
They're alone together in the house during the day, so he forgets all about her studies in order to spend every day having passionate sex with her.
But then her pregnancy leads to a friend, Sawada Kyouko, getting involved in their lives of lust and violation.

[From DLsite English]