“When guns are outlawed, only outlaws will have guns” is a popular anti-gun control saying that has been cited in print since at least 1967. The jocular anti-proverb “When marriage is outlawed, only outlaws will have in-laws” has been cited in print since at least 1979.

   
Wikipedia: Anti-proverb
Rejection: The original assertion is rejected: When marriage is outlawed, only outlaws will have in-laws.
 
6 February 1979, Seattle (WA) Times, Walt Evans column, pg. A10, col. 1:
Pat Martin of Olympia sends along this food for thought: “When marriage is outlawed, only outlaws will have in-laws.”
 
17 May 1987, Sun-Sentinel (Fort Lauderdale, FL), “Quick Draw Martinez and mushy gun law turn Florida into shooting gallery” by Steve Weller, pg. 4F:
And that story always reminds me of my favorite bumper sticker—“If Marriage Is Outlawed, Only Outlaws Will Have In-laws.” Or is it “If Guns Are ...” ah, forget it.
 
Google Books
Reality Is What You Can Get Away With:
A Screenplay
By Robert Anton Wilson
New York, NY: Dell Pub.
1992
Pg. 1:
Close ups of bumper stickers on cars:
IF GUNS ARE OUTLAWED, ONLY OUTLAWS WILL HAVE GUNS
(...)
IF MARRIAGE IS OUTLAWED, ONLY OUTLAWS WILL HAVE INLAWS
 
Google Books
Jokes, Riddles and Wisdom:
The Best Collection Ever
By Roger Kuder
Lincoln, NE; iUniverse.com, Inc.
2000
Pg. 2:
If marriage is outlawed, only outlaws will have in-laws.
   
LiveJournal
AL (twinklette) wrote in ericandannalisa,
2007-05-20 22:44:00
When marriage is outlawed, only outlaws will have in-laws
(Which isn’t completely accurate, but we’ll let it slide.)