“Why did the chicken cross the road?”/“To get to the other side” is a classic riddle from the 19th century. A bigamist/polygamist version is:
   
Q: Why did the bigamist/polygamist cross the road?
A: To get to the other bride.
   
“Why did the bigamist rabbit cross the road?” was printed in the book Waterspout Up (1975) by Anne Etkin. “Why did the bigamist cross the road?” was printed in New York magazine on July 23, 1979.
   
“Why did the polygamist cross the road?” was posted on the newsgroup alt.fan.heinlein on May 20, 1998.
   
     
Google Books     
Waterspout Up
By Anne Etkin
Baltimore, MD: T-K Graphics
1975
Pg. 34:
“Why did the bigamist rabbit cross the road?” asked Goofball in reply. And when the other rabbits just looked blank, he answered his own question: “To get to the other bride!”
   
Google Books
23 July 1979, New York magazine, “New York Magazine Competition,” pg. 82, col. 3:
Q. Why did the bigamist cross the road?
A. To get to the other bride.
Linda Bosson, NYC
     
Google Groups: bit.listserv.gaynet
It isn’t all one sided as some would believe
JOSEPH RUFFO
Nov 23, 1997, 3:00:00 AM
(...)
Q. Why did the polygamist cross the aisle?
A. To get to the other bride.=20
     
Google Groups: alt.fan.heinlein
He wrote other books
RicknMers
May 20, 1998, 3:00:00 AM
I just did a search on “Jack Mormon” and it looks like a Jack-Mormon is a non-practicing Mormon. I also found the LDS joke page. (really!) Why did the polygamist cross the road? To get to the other bride!
         
Twitter
Darragh Doyle
@darraghdoyle
Why did the Polygamist cross the road? To get to the other bride.
4:43 AM · Apr 29, 2009·Twitter Web Client
 
27 October 2009, The Sun (London, UK), “Have a Laugh!,” pg. 22:
Why did the polygamist cross the road? To get to the other bride.
   
Twitter
TheCaffeineKid
@caffeinekid
Why did the bigamist cross the road?
To get to the other bride…
5:27 PM · Jun 8, 2011·Twitter for iPhone
 
Google Books
A Circle of Wives
By Alice LaPlante
Waterville, ME: Wheeler Publishing, A part of Gale, Cengage Learning, Gale, Cengage Learning
2014
Pg. 69:
(bigamy)? And, Why did the polygamist cross the road? To get to the other bride.
 
Reddit—Cleanjokes
Posted by u/Andaree August 19, 2015
Why did the bigamist cross the road?
To get to the other bride.
   
Google Books
Dad Jokes:
The Punniest Joke Book Ever
Editors of Portable Press
San Diego, CA: Portable Press,
2017
Pg. ?:
Q: Why did the bigamist cross the road?
A: To get to the other bride.
 
Reddit—Jokes
Posted by u/rumblefish65 April 4, 2017
Why did the polygamist cross the road?
To get to the other bride.
 
Reddit—Dadjokes
Posted by u/JMClarke12 February 19, 2022
Why did the polygamist cross the road?
To get to the other bride
 
Twitter
Bob Kostic
@causticbob
Why did the bigamist cross the road?
To get to the other bride.
6:01 PM · Apr 15, 2022·Twitter Web App