Mormons are conservative, and marriage is no exception. Mainstream Mormons back traditional marriage. but when they talk about it, they often don't say "Marriage is between one man and one woman." Instead, they say "Marriage is between a man and a woman.” In Utah, that makes all the difference in the world--covers their bases with the polygamists.
Also, learn about a law that allows a polygamist patriarch to declare himself "mayor" of his own little community, with all the legal force that entails.