yeah but this guy is a broken man as a result, he doesnt want to upset the applecart because he is scared that he wont see his son if they split up which is the main reason for him staying, he also still loves his wife very much
he wants his wife to love him again and stop messing around, she even brings this guy to their house and goes out clubbing with him. he is hoping that she will get it out of her system and return to being his wife again.
I know we are conditioned to throw people out the second they cross the line but it must be more miserable to live with the grief that this sort of thing causes, at least if you split up you can try to move on.