How does one determine right social action in a society in which cultural values are diverse?

This ties in with the question, "What is moral?". If no one ever told you having premarital sex, or inbibing substances was immoral, would you think they were? No. You would not. Especially if they were socially acceptable where you lived. But there are things that we all know naturally, are wrong. And they all boil down to inflicting diliberate harm on others. In any form. So, if we all live by the principle of non agression, and tolerate the laws that others place on themselves or their people, provided they dont incite violence, and they have no right to enforce those laws on the unwilling, we could peacefully co-exist without issue.