As I thought, the article mentions WHY Mitt’s family and other Mormons originally moved down to Mexico—because they wanted to continue having polygamous marriages and the U.S. wouldn’t let them do it.
Evidently the polygamy stopped at some point, although I don’t know when.
See:
http://en.wikipedia.org/wiki/State_of_Deseret
AND
During the 1870s and 1880s laws were passed to punish polygamists, and in the 1890 Manifesto, the LDS Church banned polygamy. When Utah applied for statehood again it was accepted. One of the conditions for granting Utah statehood was that a ban on polygamy be written into the state constitution. This was a condition required of other western states that were admitted into the Union later. Statehood was officially granted on January 4, 1896.
http://en.wikipedia.org/wiki/State_of_Utah