If I was still practicing I would have no problems representing him.

There's a difference between proselytizing and responding to a conversation. I think this situation falls closer to the latter.

If your boss at work asks you what do you think of gays? You should not have to lie and should not be fired for saying gays are sinners. Likewise if your boss repeatedly starts talking about herself and her partner (totally unrelated to work) to you, I think you can argue that she is bringing any commentary about her lifestyle into the workplace and that saying you disagree with that lifestyle is fair game.