I think you handled it very well. I'm sure it caught you off guard and I can't imagine how angry you must have felt in that instant. Ideally if something like this happened to me (it hasn't yet, but I'm just a student...and as a minority I'm sure there will be plenty of opportunities to come haha) I think I would have just said something like "I don't think so" or "No, I don't think thats true". I've had SLIGHTLY offensive remarks and my reaction was to not smile and to ignore them. This at least gets the point across that they are a) not amusing b) you are not going to put up with their crap Anyways, I'm sorry you had to deal with that.