Is it actually? As far as I’m aware, it doesn’t really make any statements that anything is moral or immoral, nor is it a framework that could be used to determine such things by itself, more so a statement on the validity of such things. Or in other word, is it really a moral thesis, or is it a thesis about moral thesis?
I’ve never heard a rational defense of moral relativism that made any sense. If there are no moral truths, then serial killers have done nothing wrong for example. If a moral relativist admits that there are some moral truths, then moral relativism is completely indefensible. At that point, you’re just arguing over what is and what is not a moral truth.
How about the fact that all morals are made up and therefore obviously relative to those who made them up? There may be instinctual preference on many, but that doesn’t make it a universal rule.