The issue isn't his beliefs, it's his active attempt to limit the rights of others for no reason except for either religion or he thinks it's icky (I haven't heard any other excuses from anybody, but perhaps he has a different reason).
Regardless of what the issue is about, "his active attempt to limit the rights of others" is a legally protected activity in California. Whether we say it's about his beliefs or his activities is a moot point, since both are legally protected.
I find it interesting that so many people are trying to downplay the chilling precedent that was set here by turning everything into an argument that "he had it comin'", much like you have here. Regardless of whether he deserved it or had valid reasons for believing/acting as he did, far too many people are blindly charging ahead in their zealous fury, without stopping for a moment to recognize that they are doing so at their own peril. After all, same-sex marriage is still widely outlawed across the US (more than half the states amended their constitutions to ban it outright, and several more have laws banning it). The people cheering the demise of this guy's career seem to be blissfully unaware that the same sort of precedent could be turned against them.
Even if they aren't aware of their own peril, however, I'd hope that they would still be aware of just how scary this situation is, since it establishes that $view_we_don't_like can be grounds for forcing someone out, even if the action that they engaged in was a quiet one which they'd have no expectation would attract public attention. We ALL hold to views that are unpopular now or will be unpopular at some time, and I find it terrifying that people are defending what was effectively a lynch mob out to destroy this man and his career.
If the people cheering his demise truly believed this was about fairness, equality, or justice, they'd be going after everyone who donated $1000 to support Prop 8, not just this guy. What they did to him wasn't right, wasn't fair, wasn't just, and wasn't about fighting for equality: it was a massive display of their own failure to take the moral high ground and their own intolerance towards differing views. Sure, it may be hard to take the high ground when the "differing view" doesn't believe you should have any rights, but that's the time that it's MOST important to take the high ground.