I think maybe it’s changed (and TV ratings are different anyway) but it is/was an automatic NC-17 if you showed cunnilingus. I mean rape, murder, beating, may get you a PG-13, but muff-diving GOD FORBID you show that. I remember the director of Boys Don’t Cry being incensed that she had to cut oral sex but being allowed to show rape.
Not sure what the rules are now, since I don’t think Gone Girl was NC-17, was it?
Terry or Robyn might know better.