If a Mormon (member of The Church of Jesus Christ of Latter-Day Saints) commits polygamy they are excommunicated, period.
These criminals are no more "Mormons" than Colofornian is.
But why let telling a vile lie get in the way of titillating slander ?
Huge (false) assumption.
What you claim is like saying all Presbyterians or all Baptists or all Lutherans belong to the same denomination.
(Hint: They don't)
The fundamentalist Mormons practice & promote polygamy.
What's really strange over these years is that we've had many a FR thread discussing if Mormons are "Christian" or not...and perhaps some of the same "Mormons-are-Christians" crowd who don't like precise definitions become suddenly precise when it comes to defining a "mormon."
(Can you say r-e-l-i-g-i-o-u-s h-y-p-o-c-r-i-s-y?)