This sounds like they changed the job description in the middle of the game. If a person is against gay marriage and took the job, having no way of knowing performing gay marriage is going to become part of it, that should not become part of it. That would be like telling a witless who allows others their freedom of religion that Christmas would under no circumstances be part of the job, and then just before the big pay raises start, changing the rules so Christmas becomes part of the work requirements.
However, if at the time of employment it is made that performing gay marriage is part of the job and the person accepts the job anyways, that person has in fact accepted that requirement. Just like a person accepting a job knowing Christmas was going to be a part of the job, and then refusing to do that part and suing the company for requiring the person to do it. If the person is hired with the full knowledge that performing gay marriages would be part of the job, that person has agreed that they would do it when the need arises.
Both parties should keep their agreements made at the time of hiring. Now, if this person was blatantly and consistently infringing on others' freedom of religion, it might be acceptable payback to return this favor and require performing gay marriage anyways. But, if people respect others' freedoms, others should afford them the same.