I get where you're coming from. However, I think that in the case of Mormonism (not that I'm the most educated on the subject) you have to say that Sanderson's views are progress, of a sort. He's not saying "lets hunt them down in the streets" or any such, he's saying that in a diplomatic society he would vote for something others might find objectionable. He then proposes a solution that I think is driven by a sincere desire to find a compromise that suits everybody. He's not trying to deny gays the rights married people enjoy, he's defending the religious ideal of marriage which is under God's authority. Willingness to compromise, openness to dialogue etc make me see his personal belief system (not Mormonism as a whole, we're talking about an individual) as decidedly less than repugnant. It's certainly preferable to him attempting to force his belief system on others (voting doesn't count as forcing) which is unfortunately the tendency with many others (of all faiths/non-faiths/etc).
However like I said, I get where your coming from and its of course completely within reason for you to not financially support someone you have a strong feeling against. Objectively, his premise is certainly still flawed as minorities shouldn't have to compromise to enjoy basic rights. But its a start imo.