And when you're having sex, you're supposed to contain your passions or perhaps put a pillow over your head or perhaps a gag 'cuz otherwise you're "selfish" and such "selfishness" most likely means you're abusive.
Of course, should a child accidently come into the room because they don't know you're having sex and sees Mommy and/or Daddy with a pillow over their face or a hand or gag over their months...either they're going to think their parent(s) are trying to kill each other or that sex is something to be ashamed of.