Because we are not animals and many of us have some modicum of self-control. It seems your expectations of humans is pretty low. Perhaps I'm naive but, I would not be worried about my son banging my wife, or my wife cutting my head off -- I don't have that kind of mental energy.
Virtually all the women I interact with are classy enough not to raise me suspicion of sleeping with my son -- especially if I'm dating them. I don't have a son, but I would hope they would also respect the code implicitly.
Now you raise an issue with "class." Clearly everyone in this story were of a lower class. Perhaps in these social circles one need to worry about children hooking up with stepparents. I call this "ghetto mentality."
That you @Pioneer1 you might be watching too much step-mom porn