First of all, I think that Vera is correct that the important issues do not need to be stated in terms of different cultures. They arise within the same culture whenever people have different fundamental moral principles. There may be some issues that need to be stated in terms of different cultures, but they aren't the most important issues for us.

Second, I agree with Dee that our analysis of the meaning of ethical terms like 'good' matters here. Let me suggest that we use a definition of 'good' similar to that which David Wong proposes in his book Moral Relativity. 

Roughly, Wong suggests that "A ought to do X" means that "Under circumstances C, if A does not do X, A would be breaking a rule in an applicable moral system."

This is not exactly what Wong says. It is a bit simplified, but it allows us to speak of moral judgments as true or false in relation to a set of rules. It also allows for two conflicting moral judgments to be true at the same time. For example, the judgment that polygamy is good can be true in relation to one set of rules and false in relation to another set of rules.