Chikan Shita Joshi*sei to Sonogo, Musabori Au Youna Doero Junai

The protagonist is a run-of-the-mill businessman man in his 30s with no girlfriend. He sees a cute high school girl on the train every day. She's well developed, with nice hips, and has a healthy figure. If only he could marry a woman like her... With that figure and kind personality, she's given up struggling and gives into being molested or f*cked. That is all she's known. One day, the protagonist molests her on the train. When she doesn't struggle, he brings her to a hotel for sex. This is the first time she has had normal sex. She learns how good sex can be, and he learns about her tight ass and voluptuous tits, her family-oriented and nice personality, and how she's a dirty girl. They fall in love with each other, and their sexapades escalate.

(Source: DLsite, edited)