Are moral standards simply social conventions?
In some societies, social conventions play a large part in what is acceptable and what must be shunned. I lived in Japan for six years, a country where pagan religions and culture represent the norm. Yet, Japanese has a very strong moral code, a sense of shame, contrition, the sense of right and wrong, etc.
If you asked them why some things are "good' and some "not good" they will usually site a social convention rather than the Ten Commandments.