Here is the official definition of the PG rating. Note no mention of many specifics of what HAS to be in the movie. It's a CYA rating.
"This is a film which clearly needs to be examined by parents before they let their children attend. The label PG plainly states parents may consider some material unsuitable for their children, but leaves the parent to make the decision. Parents are warned against sending their children, unseen and without inquiry, to PG-rated movies. The theme of a PG-rated film may itself call for parental guidance. There may be some profanity in these films. There may be some violence or brief nudity. However, these elements are not considered so intense as to require that parents be strongly cautioned beyond the suggestion of parental guidance. There is no drug use content in a PG-rated film. The PG rating, suggesting parental guidance, is thus an alert for examination of a film by parents before deciding on its viewing by their children. Obviously such a line is difficult to draw. In our pluralistic society it is not easy to make judgments without incurring some disagreement. As long as parents know they must exercise parental responsibility, the rating serves as a meaningful guide and as a warning."
