Discussing any subject at the table has never been taboo for my groups. I don't think it ever should be. Personally, I think one of the worst statements about your thoughts/beliefs is that you can't discuss them reasonably with other people. If you can't make your point without devolving into a shouting match, you're not doing a terribly good job of convincing me of anything. I'm certainly willing to entertain ideas vastly different than my own, and quite honestly, a game (or any other social group) where everyone else had the same ideas as me would be boring.
I'm a Prodestant Christian and somewhat conservative in my politics. I've played RPGs with alot of other Prodestant Christians, Atheists/Agnostics, Jews, Catholics, Taoists, Buddist/Shintoists, Wiccans, and Hellenic pagans. I've played with conservatives, liberals, moderates, self-avowed communists and socialists. These aren't people that I would see at church on Sunday morning, but so what? (As an aside note, the people I tend to get along with least are other Prodestants!) D&D isn't supposed to be a forum to discuss public opinion (at least directly). Whether or not I agree with these people on on religious or political issues has no bearing on whether or not I can sit beside them to roll dice and pretend like I'm killing a dragon.
I don't like saying that we can never talk about certain things, because by barring discussion, we don't change our ideas about them. Without communicating different thoughts and ideas, we are never challenged and we never rethink our lives.