Why does Tomoya's father beat him in the novel, Earthlings?

As adults, both Natsuki and Tomoya work in Tokyo. Tomoya works at a restaurant in the city, while Natsuki works as an office temp. Tomoya's family lives in Tokyo, and he is happy to be free of their eyes when he moves into the condo in Mirai New Town with Natsuki. After he is caught asking his brother to engage in incest with him, his father takes him back to the home in Tokyo and beats him until he confesses that he has never had sex with Natsuki.