Incommensurability versus Relativism

I think many misunderstandings can emerge because of a confusion of incommensurability (read: lack of level 2 commensurability) with relativism. Cultures with radically disparate histories can arrive at similarly disparate foundational assumptions about ethics. This can result in a certain degree of cultural incommensurability, at least in the short term, but it does not follow that the meta-ethical thesis of cultural relativism is correct. The claim of cultural relativism, I take it, is that whatever a culture decides is right-for-it, so that it is inappropriate to disapprove of the practices of other cultures. This cannot be correct, if only because we naturally value not only that we practice a certain thing ourselves, but also that others practice it as well. If we are to obey the rules of rational discourse, then we do not value forcing cultures to practice our ways if we think that their intentions are good. Instead, we are simply to attempt to persuade them by means of rational argument. This is all very honourable in principle, but it will not guarantee that both sides of a cultural dispute will ever reach agreement. It is entirely possible for a true Kuhnian incommensurability to exist between cultures that will ensure that an agreement is never reached. But the justification of the moral discourse under those circumstances will still be the same as it has always been with philosophy in general. It is that the views of both sides of the argument will be clarified and deepened in the process of trying to convince someone else that they are right.