Some have said that the "Diversity Thesis" DT is a strong reason to accept moral relativism. DT states that it is an empirical fact that different cultures (or people) have different morals. This shows that ethics are relative.
But it so happens that relativism is also compatible with a Unification Thesis UT. Say that the world evolved to such a point that everyone held to the same ethical principles. That's the UT thesis. Would this prove ethics were not relative? No, the relativist could say. An example might be helpful: Currently there is a DT about which side of the road is the correct side to drive on. But we could easily become unified on this - bringing about a UT. Would this mean that there was some objective fact about the correct side of the road to drive on? No. Likewise, even if we were in an empirical situation where the UT held, the relativist need not be refuted. Relativism is compatible with both the DT and the UT.
Moreover, the DT can be accepted by the non-relativist too. Say that a secularist maintains that we need to be taught the correct ethical position, just like we are taught about math. But, due to any number of various factors, societies have slacked in their teaching duties. Or, take the Christian theist. If one combines the fallen nature of man with the creative nature of man, one could point out that not only is the DT true, it would be expected to be true in this postlapsarian state.
Thus the DT is consistent with both ethical relativism and ethical objectivism. And, it's contradictory doesn't negate either relativism or objectivism.
It seems like the DT is consistent with contrary positions.
The DT has been put forward as an argument for relativism, but we saw that its negation need not count against relativism.
And, the DT could be put forward as empirical confirmation of a system of thought that entails ethical objectivism too (in the above case of Christian theism, for example).
It seems to me that the DT can't do the work the relativist wants it to do. The DT doesn't deny ethical objectivism or ethical relativism. And, it doesn't affirm it either (i.e., since people may be mistaken; e.g., wrong answers on a math test don't prove that math is relative).
Maybe Flew, "in his prime" ;-), offers something helpful here: "If there is nothing which a putative assertion denies then there is nothing which it asserts either: and so it is not really an assertion. [...] A fine brash hypothesis may thus be killed by inches, the death by a thousand qualifications."