Honestly, I don't see any reason why a teacher needs to preserve neutrality on controversial issues--I've had quite a few teachers that expressed strong opinions on various topics, and I think it made for interesting conversations and debates. However, there is a line between "this is my opinion, let's discuss", and "this is my opinion and I AM RIGHT and will grade you badly if you disagree". However, if they do say something clearly offensive, there's no reason not to call them on it.