“Sex on television can’t hurt you unless you fall off” is an old joke, told before the invention of flat screens. The joke has been cited in print since at least season one, episode nine of the British sketch comedy television show Monty Python’s Flying Circus (December 14, 1969). The joke was also used by a television producer in 1977, when “sex on television” meant programs such as Charlie’s Angels.
 
“I hate sex in the movies. Tried it once and the seat folded up” and “The problem with sex in the movies is that the popcorn usually spills” are similar sex jokes.
         
     
The Quote Garden
Quotations about Television
Sex on television can’t hurt you unless you fall off.  ~Author Unknown
 
Wikipedia: Monty Python’s Flying Circus
Monty Python’s Flying Circus (1969–1974) was a British sketch comedy television show.
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Woman: I object to all this sex on the television. I mean, I keep falling off.
   
Monty Python’s Flying Circus
Episode Nine: The ant, an introduction
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Pepperpot:  Well I object to all this sex on the television. I mean I keep falling off. 
Shot of battered trophy.
SUPERIMPOSED CAPTION: ‘THAT JOKE WAS BRITAIN’S ENTRY FOR THIS YEAR’S RUBBER MAC OF ZURICH AWARD’
ROLLER CAPTION: ‘IT CAME LAST’
   
26 May 1977, The Plain Dealer (Cleveland, OH), “Media” by Sander Vanocur,  pg. 7C, col. 1: 
He is pleased that television critics have finally separated sex and violence. The latter, he thinks, has been over-done on television. As for sex, Schlatter said: “Sex on television is dangerous and uncomfortable. You’ll fall off and hurt your back.”
(Television producer George Schlatter—ed.)
 
Google Books
Murphy’s Law 3:
Wrong reasons why things go more!
By Arthur Bloch
Los Angeles, CA: Price, Stern, Sloan
1987
Pg. ?:
PYTHON’S PRINCIPLE OF TV MORALITY: There is nothing wrong with sex on television, just as long as you don’t fall off.
 
28 September 1987, Chicago (IL) Tribune. “From polka dots to pea shooters” by Roger Simon (Los Angeles Times), pg. 5:
Best slogan of the month: There is nothing wrong with sex on television as long as you don’t fall off.
 
7 August 1989, Fayetteville (NC) Observer, “Maternity BDUS?”:
There is nothing wrong with sex on television as long as you don’t fall off.
 
20 May 1991, St. Petersburg (FL) Times, “A bumper crop of words to live by” by Mike Harden, Floridian, pg. 3D:
“Sex on television can’t hurt you unless you fall off.”