So they can have sex with him while he's asleep. They do this several times. He suspects something is happening, so he places a camera in his room and finds out the truth. The reason is because mom is traumatized so she can have sex only when the male is helpless. He has sex with mom at the kitchen, and at the end they tie him to a chair so mom can have sex with him while feeling safe.
It's a really old story, but perhaps somebody remembers it. TIA.
