Marriage has been practiced between one man and more than one woman many times throughout history, including in the Bible by biblical patriarchs. There were times when God accepted it. We believe that most of the time he rejects it, and we even have scripture that states as much. See Jacob 2:24-30.
For further reading, see:
http://en.fairmormon.org/Mormonism_and_polygamy/Book_of_Mormon_condemns_the_practice