Yes.
Jack Valenti created the rating system as a replacement for the Hays Code, which he (and nearly everybody else in Hollywood) detested (and which had become a major obstacle to keeping theatrical films competitive with TV).
His 1968 design (originally G, M, R, and the non-copyrighted X; M became GP in 1970, then PG in 1972) was uninformative by design: the only qualification for being on the ratings board was that one had to be a parent, the sole criterion for the rating was suitability for children, and the rating said nothing about whether sex, violence, horror/shock, or something else entirely was the primary reason, in order to avoid spoilers.
And it worked. At first. (And yes, as a matter of fact, I am deliberately quoting "Patterns of Force") The original Airport got a G, and nobody mistook it for kiddie material. And A Clockwork Orange got an X, even though, from what little I know of it, you'd have to be pretty sick to get horny from watching it.
There is no doubt in my mind that Valenti assumed that nobody would deliberately seek a more restrictive rating, and that most films would be released with a G rating, most of the rest with an M/GP/PG rating, and that only those films for which content essential for telling the story demanded an R or an X would be released with those ratings. He almost certainly didn't envision low-budget films intended for very young, minimally-supervised children (and far too saccharine for anybody else) to take over the G rating, and he certainly didn't envision hardcore pornography ever making it out of the back rooms of adult book shops.
The very first time I was even aware that the rating system was starting to break down was back in the 1970s, a line in a TV sitcom (I think it was Maude, and I think it was Maude's grandson talking to his mother), voicing an objection to "some dumb movie rated G for kids"). Although by then, I'd already been surprised by the PG rating given to The Sting; I'd enjoyed "A Piece of the Action," and always liked ragtime music; it looked like it would be interesting (ironically, I still haven't bothered to see it all the way through). And years later, I was surprised by SW and CE3K both getting PGs, and later on by Spielberg apparently including the phrase, "penis breath" in ET, apparently purely to avoid any possibility of a G rating.
In the interest of full disclosure, the very first movie I saw in a walk-in theatre was Disney's The Million-Dollar Duck, on a double-bill with The Barefoot Executive. And the next time I saw a movie in a walk-in theatre, it was the MGM retrospective, That's Entertainment.
