I'm calling it now: he's a widower who started watching miyama's porn because she resembles his late wife, and it became a way for him to cope with the loss.
bonus theory: his wife and tamagawa are related somehow, and that's why they look similar.
I guess I'm the only one who think Manager like Miyama because she reminds him of his former wife who (I assume) died and Miyama is cope from losing wife. I thinks more tragic than broken marriage due to porn addiction.