As far as has ever been clarified to me, the exact rule is no muzzle, and no non-human dick. I would say to avoid anything that is clearly just anthropomorphic, and actually is more human in design.
I can say that I have seen demons, naga, and sea creatures like mermen without issue, but I have also seen people freak out at satyr and say that centaur are perfectly fine. It really is hit-or-miss at times.
But everyone seems to agree that non-human dicks and non-human mouths are unacceptable.