“Forbidden fruit” refers to the biblical story of Adam and Eve eating the forbidden fruit from the Tree of Knowledge. “Forbidden fruit creates many jams” has been a roadside sign message at many churches since at least 1998. The saying was popularized in the title of Mary Katherine Compton and David Compton’s book, Forbidden Fruit Creates Many Jams: Roadside Church Signs Across America (2001).
     
 
Wikipedia: Forbidden fruit
Forbidden fruit is a metaphorical phrase that originates from the Genesis narrative concerning Adam and Eve in Genesis 2:16–17. In the narrative, the fruit came from the Tree of Knowledge of Good and Evil and was eaten by Adam and Eve in the Garden of Eden. As a metaphor, the phrase typically refers to any indulgence or pleasure that is considered illegal or immoral. It may also refer to something potentially dangerous or harmful, particularly relating to human sexuality.
   
31 January 1998, Daily Press (Newport News, VA), “At Week’s End,” pg. A10:
We had a chuckle, at what we thought was the president’s expense, when we saw this sign: “Forbidden Fruit Creates Many Jams.”
 
Surely, the sign at the Isle of Wight County church must be a dig at Bill Clinton, as he agonizes his way through the Monica Lewinsky scandal. Wrong, said the Rev. Bill Fulcher, pastor of Faith Free Will Baptist Church on Carrollton Boulevard.

Fulcher said he’d seen the same phrase on a church sign in North Carolina. He’d assembled his sign several days before the alleged sex scandal became public. Yet, he acknowledges, different people can draw different conclusions. “It strikes the eye, and causes people to think,” he said.
 
Google Books
Time-Saving Ideas for Your Church Sign:
1001 Attention-Getting Sayings
By Verlyn D. Verbrugge
Grand Rapids, MI: Zondervan Publishing House
1999
Pg. ?:
Forbidden fruit creates many jams
 
Google Books
701 Sentence Sermons:
Attention-Getting Quotes for Church Signs, Bulletins, Newsletters, and Sermons
By L. James Harvey
Grand Rapids, MI: Kregel Publications
2000
Pg. 112:
FORBIDDEN FRUIT CREATES MANY JAMS
   
30 March 2000, Daily Globe (Ironwood, MI), “Actual Signs Outside Churches,” Porcupine Press, pg. 28, col. 3:
Forbidden fruit creates many jams.
 
OCLC WorldCat record
Forbidden fruit creates many jams : roadside church signs across America
Author: Mary Katherine Compton; David Compton
Publisher: New York : New American Library, ©2001.
Edition/Format:  Book : English
 
OCLC WorldCat record
Forbidden fruit creates many jams
Author: ResiNation (Musical group)
Publisher: Canada : Modular Mood Records, 2002.
Edition/Format:  Music CD : CD audio : Popular music : English
 
OCLC WorldCat record
Life is short, pray hard : forbidden fruit II : more roadside church signs across America
Author: Mary Katherine Compton; David Compton
Publisher: New York : New American Library, 2003.
Edition/Format:  Book : English
   
Google Books
The Telling: a novel
By Mike Duran
Lake May, FL: Realms
2012
Pg. ?:
Across the street stood the Vermont. Newly posted signs announced that the old playhouse was under construction, and the marquee now promised a “Word for the Day,” today’s message being: FORBIDDEN FRUIT CREATES MANY JAMS.