Jennie Gerhardt is a destitute young woman. While working in a hotel in Columbus, Ohio, Jennie meets George Brander, a United State Senator, who becomes infatuated with her. He helps her family and declares his wish to marry her. Jennie, grateful for his benevolence, agrees to sleep with him. He dies before they marry, and Jennie is pregnant.
She gives birth to a daughter, Vesta, and moves to Cleveland with her mother. There she finds work as a lady's maid in a prominent family. In this home, she meets Lester Kane, a prosperous manufacturer's son. Jennie falls in love with him, impressed by his strong will and generosity. She leaves her daughter in Cleveland and travels to New York with Kane. He does not know of her illegitimate daughter and wants to marry Jennie. But because of their difference in class, he anticipates his family's disapproval and decides to take her as his mistress.
They live together successfully in the university neighborhood of Hyde Park, Chicago. After three years, Jennie tells him that Vesta is her daughter. Kane does not yield to his family's pressure to leave Jennie. But, after his father's death, he learns that his inheritance of a substantial part of the family business is conditioned on his leaving her. On hearing the will's terms, Jennie demands they separate for his sake.
During their trip to Europe, Kane meets Letty Gerald Pace, an affluent widow. Bowing to pressure from Jennie and his family, he decides to marry. After providing financially for Jennie, he marries Letty, resuming his former social status. Jennie loses her daughter to typhoid fever and adopts two orphans. She continues to love Kane.
He becomes seriously ill and tells Jennie he still loves her. She tends him until his death, and mourns secretly at his funeral.

Question: Why would Kane's family disapprove of his relationship with Jennie?
Answer: Because they are of different class
Alternative answer: She is from a lower social class.