Mama! Tsuma?

The protagonist, Makoto Sakurai, is a high school student. One day, his new neighbour, Atsushi Uchiyama, tells him an shocking secret about his family: he has a secret sexual relationship with his own mother. For him, it's completely normal. But ever since, Makoto's way of looking at his mother, and at his friends, has changed. Then, one day, Makoto's father has to leave for an extended business trip, leaving Makoto alone with his mother. Makoto feels something vaguely unsettling, and his concerns turn out to be a reality. That day, when he comes back from school, his mother is waiting for him completely naked, ready to start an incestuous relationship with her son. And it looks like his friends are in a similar situation!