This view constitutes an anthropological description, and denies objective morality on the basis of a recognition that different cultures exhibit different moral stances. While this view may sound valid, it has at least two problems, one of which (the second one I'll discuss) is fatal.
First, it is not clear that different cultures really do have significantly different value systems. In most apparent cases of conflict, it is facts--and not values--that provide the differences.
Even in the highly charged abortion debate, all the parties share a value system that says that taking innocent human life is wrong. The debate turns on a question of facts, the issue being when does an innocent human life begin?
We believe that cannibalism is wrong, but that other animals are legitimate sources of food for us. In India, by contrast, it is taboo to eat beef. Big difference in moral values, right? Well, not really. You see, in India, they believe that the cow in question may be the reincarnation of a human being, and so their refusal to eat beef has its foundation in the same morality that prevents us from cannibalism.
Well, then, what about human tribes that really do practice cannibalism? As it turns out, cannibalism doesn't come naturally to these tribes, and learning such a practice involves a period of great emotional stress for the children of each new generation. In other words, even where the behaviors contradict more conventional mores, those behaviors are inculcated with difficulty for the very reason that they go against what seem to be universal moral norms among humans.
The second problem with descriptive cultural relativism is that it involves a fatal logical flaw. The argument looks like this:
Premise: Different cultures exhibit different moral codes.The argument is fallacious, as it involves a non sequitur. The conclusion doesn't follow from the premise. This can be seen by a similar example. Let's say that my wife and I seperately attempt to compute our income tax liability, and we come up with different answers. It would be tempting to conclude that this indicates that there is no correct answer (and that, therefore, the IRS is asking us to do the impossible, and we are justified in not bothering with taxes this year). Obviously, the conclusion is wrong. There is a correct answer, even if one or both of us failed to arrive at it. In the same way, the observation that different humans or human groups do not exhibit or even profess the same moral code does not in any way demonstrate that there is no universal moral law.
Conclusion: There is no single morality that holds for all human beings.
Descriptive cultural relativism is logically flawed and an inaccurate assessment of the true situation with regard to morality.