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The LDS church's stated reason back in 1890 for stopping the practice of polygamy, was because it was against the law of the land. The Federal government was coming down hard on the Saints back in the late 1800s.
In today's world, there is no way the church wants polygamy to EVER become legal, and it very will could be if gays are allowed to marry. It would be a PR nightmare and would open up a can of worms that the church doesn't want to open.
Backing an amendment defining marriage, is in effect tying G-d's hands should He decide it is time to restore the practice of Celestial plural marriage.
Talk about hubris and a total disregard for agency....