There are cultural norms that directly contradict and claim the other is immoral.
Architects do not claim their style is the only right way to do it rather they claim the principles of architecture are right and that code is loose enough to have all those styles be permissible.
It's possible for a culture's meta-ethics claim that its norms are
exclusively correct to be wrong, and yet its norms could be one of many correct systems. If Frank Lloyd Wright had claimed that his buildings were the only good buildings, that claim could be rejected without denying that his buildings (along with many others) are good. That seems to be a common situation in applied sciences — more than one solution is possible.
Ignorance of a law is no excuse in the US though inability to understand a law is an excuse in the US. This is how the US regards Legal Competency. If we assume the same logic is true for Moral Competency then incorrect beliefs would not be Moral Incompetence but inability to understand morality would be Moral Incompetence.
You must be allergic to anything social. Unfortunately, morality is social through and through, to its very core. Society precedes individual conscience. The individual can only develop as a complete human being with the support and guidance of other humans. Social norms are a necessary starting point for practical morality. Social norms can and do change, as people experience problems with the existing system.
I don't consider legal competence and moral competence to be parallel systems. I consider legal competence to be a minimum for a normal adult human. People can and do exceed that minimum by noticing problems in the laws and reforming (or occasionally even overthrowing) them.
Rationality is important, but more important, in my view, is empathy. When people are only motivated by their narrowest interests, they multiply conflicts and commit atrocities. When people have concern for others as well as themselves, they are motivated to make arrangements that benefit everyone.
I see no reason to legislate Morality.
Can you point to anything as an example of how people can live without laws (or only with laws that have zero moral content)?