I agree, we're making the same general point -- it does seem crazy to ask someone to "ensure they hold no offensive opinions". So why, for example, should the opinions of the person who wants to speak their mind about the immorality of gay marriage get precedence over the opinions of the person who thinks they should be allowed to get married?

I agree with that. "Don't go around pissing off co-workers" is a sensible rule. But it's also our responsibility to not look for reasons to be offended. If you ask me about something I'm not going to lie. But I'm not going to shove it in your face either, if I think it might offend.

I’m an atheist. I’ve worked with people who thought I’m going to hell. What am I going to do, get them all fired?

No. Just be polite and respectful to your colleagues. Not every stone needs to be turned.

And it is your right not to work with that person, whether that means you choosing to leave or take internal action depending on circumstances (e.g. threats). However, who is entirely exempt from ostracizing others for their beliefs? For every person there is some universe of beliefs that are seen as such a threat to their reality, principles, or well-being that they will fight those beliefs and their representatives. That universe of beliefs may differ in size or relative merit on the basis of evidence for each person, but it exists.