It's an interesting idea, but I doubt that it will accomplish anything, especially when you consider that the majority of gay people in America probably won't do this because they know it won't do anything. Unlike the immigration issue (which they are obviously stealing the "call out "sick" idea from), people have a moral objection to gay marriage, calling out one day is not suddenly going to change people's minds about it. It also doesn't make sense, what does calling out have anything to do with gay marriage? The whole reason the illegal immigration movement called for a day where immigrants wouldn't work was to show that immigrants contribute to the economy, no one is questioning whether gay people contribute to the economy, so I just don't see the connection. Maybe they are trying to show that there are more gay people in the community than people think? Seems like there are more productive ways to do this (and that still wouldn't change most people's minds).
You really want to combat Prop 8, focus on the outrageous claims the pro prop 8 campaign made. Things like Prop 8= religious freedom, and prop 8 = tolerance. That would be far more productive in advancing gay rights than calling out of work.