Again a bit extreme because someone doesn't support gay marriage, people resigning from their posts, Facebook campaigns etc are all designed to either get this guy to publicly announce he has changed his views (not freely) or resign/have him fired. There are only two options for him, this is wrong on every level. And what they are saying is important too, they are not simply disagreeing with him, like you suggest, they are not just voicing their opposing opinions, like you suggest, they are actively attempting to force him to change opinion or have him fired. It's my way or the highway, why you are defending such actions is beyond me.
You seem to keep choosing to ignore the bit where he funded a bill to enact a discriminatory anti-gay marriage law.
It's not just his opinion; he apparently believes in actually discriminating against gays in this way.