Because they have institutionalized protection and they know it. The lefties have to pause and think about who's higher on the oppression scale to decide who they should side with, but even if the Muslims lose this one, it's not like they're going to be slandered and criminalized like whites and conservatives would. So they have nothing to lose protesting. And if they win, then it gives them a farther foot in the door to attack even more institutions (and not just the gays) and exert their influence even more.