Just to set the record straight about older movies, when Hollywood adopted the Production Code (better known as the Hays Office, after the man running it), the whole idea was that every movie should be suitable for everyone. If a film failed to received the Motion Picture Code approval, no theatre would handle it. (Films that were really aimed at adults were labeled "educational" and showed in traveling tents. One such film was Reefer Madness, which most of you have heard of.)
At any rate, Gone With the Wind, released in 1939, broke the rule with Rhett Butler's line. An altenate scene with Butler saying, "Frankly my dear, I don't care," was shot, but David O. Selznick, the producer, thought that it didn't pack the same power, and wasn't used. The Hays Office, facing a blockbuster that the whole country was waiting for, couldn't block the film, but did give Selznick a $5,000 fine.
Flash ahead to the early '50s, when The Moon Is Blue was released with the word "virgin" in it. That shocked people.
Another facet of the Code at that time was you could not get away with a crime. This included adultery (check out the ending of Anna Karenina, with the count feeling that he has been punished for all time) and homosexuality, which couldn't even be mentioned. (For that, check out the film version of Advise and Consent.)
The code broke down in 1966 (I think that was the year) with Who's Afraid of Virginia Wolfe? which featured expressions like "hump the hostess" and a dance between Elizabeth Taylor and George Segal that mimed sex. The Code decided to explicitly say that some movies were not for children, and began to tag films SMA (suggested for mature audiences).
This only lasted a few years, when they came up with the current system. There were, at first, only four ratings - G, M (which became GP then PG), R and X. One of the first films to get an X rating was Last Summer, which definitely was for adults, but got an X instead of an R because of a scene where two boys were discussing what sex a pigeon was. They decided that it was male because it had a male sexual organ - they used the term that many people use for a needle stick.
Another film a few years later, Harry and Tonto, received an R because a character referred to a female relative by a four-lettered term for female gentilia.
Probably since the release of All The President's Men, langjuage is taken mostly on individual cases, whether or not it is germane to the story line.
By the way, PG-13 was introduced after the release of Indiana Jones and the Temple of Doom, which was too intense for a PG, but that was all they could give it.
Sorry if I've gone on too long, but some of the arguments posted were making statements about film history, and I just had to chime in.