It is why incongruity should give us pleasure, and why some sorts of incongruity prompt laughter and others (such as Oedipus' parenthood) do not.

Perhaps, but consider Oedipus Rex by Tom Lehrer:

  There once lived a man named Oedipus Rex,
  You may have heard about his odd complex.
  His name appears in Freud's index
  'Cause he loved his mother.

  His rivals used to say quite a bit
  That as a monarch he was most unfit
  But still in all they had to admit
  That he loved his mother.

I don't care who you are, that's funny.