Sorry if my english is a bit rusty, so this past week I've been struggling to find this specific doujin but I can't find it, I can't quite remember the start of it, mostly of the story the affair is kept hidden from dad who at a certain point in the plot open the door of their room and stares at them doing it, says nothing and just leaves, the other two just keeps going and later on dad and mom divorces but he just accepts the relationship, almost in the end of the story dad and son met at a restaurant and talk about it and dad has not much of a trouble from his own son just having her mom, if it helps other thing I remember from the plot are the mother going at a ex-students meeting from high school iirc or something like that, and the father smoking in the meeting at the restaurant with his son