My brother (convert to LDS) and his wife are canvasing their neighborhood in support of Prop. 8, as are many in his ward. You have to give credit where it is due. When it comes to getting its members out onto the streets, the LDS church is pretty solid.
However, my own (Catholic) parish is also extremely involved in both Prop. 8 and Prop. 4 (the abortion/parental notification law). And the Knights of Columbus organization has funded radio spots supporting the Prop. 8 measure as well. “Vivat Jesus!”
Personally, I think that the opposition focuses on actions by the LDS Church and Catholic Church because (in their minds) the focus on these institutions tends to portray the pro-marriage argument as nothing but “extremist” religion to the mainline protestant denominations and non-christians which make up a majority of the registered voters. These latter groups are, of course, already generally non-opposed to gay “marriage.”
Peace,
-Robert