A jocular explanation of the movie rating system has been printed on several images. It was explained like this in February 1973:
 
“If a movie is rated G, the hero gets the girl; if R, the villain gets the girl; and if X, everyone gets the girl.”
 
It was explained like this in October 1974:
 
“I finally figured out the movie ratings, GP is where the good guy gets the girl, R is where the bad guy gets the girl, and X is where everybody gets the girl!”
 
Both explanations are from unknown authors. American actors Kirk Douglas and his son, Michael Douglas, have been credited since about 1988, but they did not originate the saying.
 
   
Motion Picture Association of America film rating system
MPAA film ratings
Since the late 1990s, the MPAA film ratings have been as follows:
 
G – General Audiences
All ages admitted. Nothing that would offend parents for viewing by children.
PG – Parental Guidance Suggested
Some material may not be suitable for children. Parents urged to give “parental guidance”. May contain some material parents might not like for their young children.
PG-13 – Parents Strongly Cautioned
Some material may be inappropriate for children under 13. Parents are urged to be cautious. Some material may be inappropriate for pre-teenagers.
R – Restricted
Under 17 requires accompanying parent or adult guardian. Contains some adult material. Parents are urged to learn more about the film before taking their young children with them.
NC-17 – Adults Only
No One 17 and Under Admitted. Clearly adult. Children are not admitted.
(...)
From M to GP to PG
In 1970 the ages for “R” and “X” were raised from 16 to 17. Also, due to confusion over whether “M”-rated films were suitable for children, “M” was renamed to “GP” (for General audiences, Parental guidance suggested), and in 1971 the MPAA added the content advisory “Some material not generally suitable for pre-teenagers”. In 1972, “GP” was revised to “PG”.
 
IMDb (The Internet Movie Database)
Kirk Douglas : Quotes
If the good guy gets the girl, it’s rated PG; If the bad guy gets the girl, it’s rated R; and if everybody gets the girl, it’s rated X.
 
8 February 1973, Wakefield (MI) News, “This and That,” pg. 1, col. 1:
If a movie is rated G, the hero gets the girl; if R, the villain gets the girl; and if X, everyone gets the girl.
 
5 March 1973, Akron (OH) Beacon Journal, What Movie Ratings Mean” by Kenneth Nichols, pg. B22, col. 1:
ANGE CIRILLI is a pizza baker but he has the best explanation we’e yet heard of the rating system for movies.
 
“If a movie is rated G,” Ange says, “the hero gets the girl; if R, the villain gets the girl; and if X, everyone gets the girl.”
 
24 March 1973, Omaha (NE) World-Herald, “A Smile or Two,” pg. 6, col. 2:
If a movie is rated G, the hero gets the girl; if R, the villain gets the girl; if X, everyone gets the girl.—Iron County, Wis., Miner.
 
20 April 1973, Indianapolis (IN) Star, Earl Wilson syndicated entertainment column, pg. 17, col. 2:
TODAY’S BEST LAUGH: Phil Wise explains the movie rating system: “If it’s rated G. the hero gets the girl; if it’s R, the villain gets the girl; if it’s X, everybody gets the girl.”
     
12 October 1974, The Register (Iola, KS), pg. 8, col. 1:
I finally figured out the movie ratings, GP is where the good guy gets the girl, R is where the bad guy gets the girl, and X is where everybody gets the girl!
 
22 November 1987, Newsday (Long Island, NY), “Don’t Forget to Miss This One!” by Mike McGrady, pg. A11, col. 2:
PUBLIC SERVICE ANNOUNCEMENT: It seems to me that periodically we should explain the complicated movie rating system. I’ve always liked Kirk Douglas’ explanation: “I’ve finally figured out today’s movie rating system. ‘G’ means the hero gets the girl; ‘R’ means the villain gets the girl; ‘X’ means everyone gets the girl.”
 
20 November 1988, Sunday Republican (Springfield, MA), “Glad You Asked That!” by Marilyn and Hy Gardner, pg. F-2, col. 4:
A DEFINITION OF MOVIE RATINGS BY KIRK DOUGLAS:
“‘G’ means the hero gets the girl. ‘R’ means the villain gets the girl. And ‘X’ means everybody gets the girl!
   
Google Books
Bulletin with Newsweek (Sydney, Australia)
Volume 110
1988
Pg. 123:
Thespianic flavor of the year Michael Douglas has been casting his eye over the film certificate system and claims to have unravelled its mysteries. He tells me: “PG means the hero gets the girl, M means the villain gets the girl and R means everybody gets the girl.
 
Google Groups: rec.arts.movies
WORST ACCENT IN FILM
Dragan Antulov
4/2/95
(...)
Movie rating system: Rated G : Nobody gets the girl.
Movie rating system: Rated PG : The good guy gets the girl.
Movie rating system: Rated R : The bad guy gets the girl.
... Movie rating system: Rated X : EVERYBODY gets the girl.
 
Twitter
Wylie Jones‏
@Wylieknowords
“G means the hero gets the girl. R means the villain gets the girl. And X means everybody gets the girl.” Kirk Douglas     http://www.knowords.com
6:31 PM - 15 Nov 2009
 
Google Books
The 2,548 Wittiest Things Anybody Ever Said
Selected and compiled by Robert Byrne
New York, NY: Touchstone
2012
Pg. ?:
1,392
When it comes to movie ratings, G means the good guy gets the girl, R means the bad guy gets the girl, and X means everybody gets the girl.
—Kirk Douglas
 
Google Books
The Very Best of Kirk Douglas:
Thoughts of a Hollywood Legend
By David Graham
Published by author
2014
Pg. ?:
* If the good guy gets the girl, it’s rated PG; If the bad guy gets the girl, it’s rated R; and if evverybody gets the girl, it’s rated X.