Because of their support of the ballot initiative. Apparently they aren’t able to get over their loss and accept the will of the people:

More than 3,000 protesters marched near the landmark Los Angeles California Temple of the Church of Jesus Christ of Latter-day Saints in Westwood today to protest the church’s strong support for Proposition 8. Santa Monica Boulevard is closed and nearby traffic is gridlocked.
Opponents of Proposition 8, which outlawed gay marriage in California, were waving banners and chanting. A similar protests occurred on Wednesday night in Hollywood and West Hollywood.

I wonder why they aren’t protesting black churches? Blacks are the ones that pushed the measure over the top, without the Obamanites, the measure wouldn’t have passed.
Protest black churches? *snicker* That will never happen, they don’t have the guts to do it. It’s easier to beat up the Mormons, no one will call them bigoted for doing so (except maybe conservatives and religious people who understand what’s going on here — the left doesn’t want us to bring out religion into the voting booth).