The passage of Proposition 8 in California, which categorically defines marriage as being only between one man and one woman, was successful in part because of the LDS (Mormon) Church's mobilized support for the issue, despite Mormons being only two percent of the state's population.
According to the Los Angeles Times, monetary support for Proposition 8 from Mormons is estimated to have been about twenty million dollars. Donations from individual Mormons were in response to a letter from church leader read in all LDS congregations in California, urging members do all you can to support" the proposition by donating "your means and time." The church's position, the letter said, was that "marriage between a man and a woman is ordained of God and the formation of families is central to the Creator's plan for His children."
As well as providing financial support to Prop 8, Mormons also hosted websites, formed an outreach organization: Protect Marriage Coalition, posted pro-Prop 8 videos online, and so on.
I think the Mormon stance against same-sex marriage is ironic, considering that the US government forced the LDS church to give up polygyny in 1890, as a pre-requisite to Utah becoming a state.
Perhaps if the US had allowed polygyny and other forms of polygamy to remain legal and aboveboard, we'd not be seeing these renegade Mormon groups forcing underage girls to marry old men, as the government could have applied the same rules to such marriages as they do to monogamous ones (consenting adults only).
One would think the Mormons would be especially sensitive to marriage discrimination, but apparently this isn't the case.