No group is more emphatically and publicly opposed to the practice of polygamy than the Latter-day Saints. The topic is, however, irresistible and perennial. While the Mormon Church banned plural marriage more than 100 years ago and promises excommunication to those who practice it, its spokespeople find themselves having to explain polygamy’s legacy over and over to reporters who watch “Big Love” or are curious about Mitt Romney’s ancestry. “I wish to state categorically that this church has nothing whatever to do with those practicing polygamy,” said LDS president Gordon B. Hinckley more than a decade ago.