It appears that all ethical theories are vulnerable to the challenge that their values, whatever they are, are products of cultural relativity. Since nobody can transcend his/her culture, is the only answer to shrug our shoulders and say, "So what? In this culture X is right and Z is wrong, at least for now. So for the time being, respect our current code or risk the consequences of not doing so."?
One can perhaps say a similar thing re all attempts to gain knowledge: "what we judge to be true of the world is ultimately a product of our (current) standards for gaining knowledge, the theories/procedures we currently judge to be best. Those standards may be relative between cultures, and almost certainly are relative between different times/eras." But here we typically DO say "so what?", in effect: we believe whatever our current best theories tell us to believe, recognizing that later theories may tell us to believe something else. And if that's good enough for knowledge in general, why shouldn't it be good enough in ethics as well? (I take your phrasing of your question to suggest you don't think the 'so what?' answer is quite good enough; my response is meant to shore it up a bit by noting that it's all we ever have, in general, so it better be good enough or else!) hope that's useful. ap