I agree with most of the things you are saying. I would only like to point out a few points which I believe you should agree with.
If some morals are relative to culture, this does not mean that all morals are relative to culture. Meaning that if dressing inappropriately in Saudi Arabia is considered immoral while in America it is perfectly fine, this does not mean that all questions of morality depend on culture.
Would you not agree that their are absolute moral principles that can be verified regardless of time, individual, culture? I cannot prove this is the case as no one can prove that ethical relativism is the right philosophy but basically things like raping a child are fundamentally immoral regardless of any of the factors I have mentioned above.
maybe but either way we cannot know if this is the case or not.
The problem is not of whether there is the absolute principles that you are talking about... the problem is that of judgement.
We cannot judge objectively whether there exists such laws or not because our judgment is always shadowed by past experience and emotions.
If these laws were self evident then this problem is already solved but they are not.
Attaining knowledge of such thing is impossible because making an objective judgment about it is impossible... so believing it exists or not is impossible.
Moral relativism does not suggest that consensus about a moral topic is impossible because this would obviously be false as the world does agree on many things. However, Taking the exact same factors into account while performing the judgment (even if agreement was met) is impossible and that is why they claim its relative to the judge.
On the other Hand, Mathematics does not take these factors into account because the process of judgment in Math does not take personal emotions and culture into account. It is none debatable unlike morals.
So the point is, that maybe moral laws are objective in themselves (meaning there is a best way to act in each situation) but we as human beings cannot objectively know their existence, it is above our capabilities and hence we disagree on this very topic. This too has to do with faith that is why i don't see it as sufficient proof for god's existence. If you were to prove its objective reality using an objective argument like that we use in math then it would be a proof of god's existence.
Humans then pass judgment using their moral books on other society's moral books and so cultural shocks take place. So in attempting to reach agreement on a wider scale, while acknowledging that our judgment is relative, groups such as the UN and others (in this age of globalization) try to make morality look more like math to search for an objective moral truth like the one you believe in. They add criteria to it to have a more efficient judging system so that we can be able to punish and reward while maintaining justice. Criteria like Universality where you take a certain situation and analyze a certain action by seeing if every person in the world were to do that action in that certain situation. On that wide scale, a universal scale, would the world be better with slavery or without?
The world agreed that slavery was not the best way to go like 100 years ago only. This is because of our refining criteria that tries to eliminate the subjective factors judges take into account making morals more like math in order for the world to live a better life.. to making a better state.
Whether the world at one point will Fix its moral code as THE CORRECT MORAL CODE (like the one you suggest) that will not be modified ever again is a matter of faith.
shit you make me write a lot.
EDIT: That is why morality is not a science... and that is why whenever we try to objectively study something we try to turn it to numbers.. as numbers are the most objective thing we have.