Alasdair MacIntyre convinced me of why modern approaches to ethics are broken and contemporary ethical debates interminable. Classical ethics, to paraphrase him, was part of a three-legged stool: the actual, current human state, the human telos (or the proper goal of human life), and ethics, which was guidance on how to move from the current state towards the goal.
With the Scientific Revolution, however, the Aristotelian baby of ethical teleology was thrown out along with the bathwater of Aristotelian physics. As a result, we were left with a current state, and a set of instructions. Instructions for what? Hmm, no one quite knew. How do we tell if those instructions are working? Again, it wasn't clear.
Thus, we experienced an onslaught of attempts to make sense of those instructions, to revise them according to some novel interpretation of what they were for, and so on. It's as if we were in the situation of a people whose ancestors had left them detailed instructions on the best ways to get to New York, which they still held to be very important, even though it had become an article of faith for them that there is no such place as New York!
And this analogy explains why it is true that morals are both relative and objective, and why it is so off target to accuse anyone who points out the contingent, situated nature of ethics as holding that "hey, anything is as good as anything else!" After all, the way in which it was best for an ancient Athenian to try to reach New York (involving heading west, sailboats, a gradual exploration of the Atlantic, etc.) is very different from the best way for a current resident of Tokyo to get here, perhaps by booking a plane ticket. And yet that doesn't in the least mean that we can't say that lying down in bed, or burrowing towards the center of the earth, are very bad ways of heading towards New York, and the ways mentioned previously are objectively better.
And so, when I was noting the culturally situated character of all ethical reasoning, it was not only uncharitable but also rather obtuse for a critic to say, "So, you think there was nothing wrong with the Holocaust, since that was OK per the Nazis culture!" Well, no, son, the Holocaust was a lot more like burning down all the vehicles in the area, killing the drivers, hacking off one's own arms and legs, and then screaming in rage that the drivers had betrayed you.
And that, we can see objectively, is not much of a way to get to New York.