Actually polyandry WAS practiced in early Mormonism.
http://mccue.cc/bob/documents/rs.early%20mormon%20polyandry.pdf
http://www.timesandseasons.org/?p=3772
Joseph Smith was married to several women who were already married.
In fact Apostle Parley P. Pratt who was murdered in Arkansas was accused of polyandry and killed, by his new wife’s current husband.