I think (personally) a lot of this falls in the question of Focus. The example of common morality is quite unanimously accepted on topics such as murder, raped, etc. If you widen the focus from what, and into the Why, the common ground with some will fall apart, but it is nonetheless still a discussable topic, and some learning will take place. Going into details of how, when, who will probably disintegrate more of the "commonality", and much can be learned from such discussions, but the common ground remains in the what. (such as murder is Immoral)