Mormons haven't practiced poligimy since the 1831. To be precise, in the Doctine & Covanents, section 41, (revelation given to Joseph Smith, Sidney Rigdon, Parley P. Pratt, and Leman Coplet at Kirtland Ohio in March 1831) verse 16: "Wherefore, it is lawful that he should have one wife, and they twain shall be one flesh." Poligimy, was actually not even a "church practice." It was not taught in the church, although people did it. Not only mormons. Please, don't get me started. Just a word to the wise. If you have a joke that refrences mormons, be careful.
H-D