I don't see how it can be ethically sticky. Suppose an interpreter was requested at a KKK rally? The interpreter has the right to refuse. I used an interpreter today and we were discussing things, and one thing she told me was that she refuses to interpret for a specific individual. I don't see the problem.
Well, I don't know exactly how the ethics of the terps are played out. But I do know that I do not have the right to refuse services to a client simply because I disagree with their religious beliefs, their political beliefs, their racist attititudes, their homophobia, etc. I am bound by my professional ethics to remain objective and to offer the best treatment possible for the client in spite of my personal position.
However, if I find that the disagreement is beyond my personal capability to put aside in order to be completely objective, then I must refer to another therapist so that continuity of treatment is not interrupted. Then I am also ethically bound to enlist supervision to help me work through my personal issues that are preventing me from being objective.
So, yeah...I have the right to refuse but I also have the obligation to insure that the client is provided for.