Not all mormons are poligamists. Poligamy is pretty similar to incest in the frequency it occurs. They are both illegal and they both happen, but they aren't wide spread.
Poligamy is something mormons USED to do, now most mormons don't agree with it. Only the really hardcore fucked up mormons do.
Actually, polygamy is encouraged among FLDS (Fundamentalist Latter Day Saints, or, as I like to call them, Super Mormons) as is digging up dead bodies to baptize them.
Regular LDS/Mormons aren't that crazy, but they're still odd (specially with their special underwear).