Cultural relativism is certainly a valid school of ethical thought — one with which I happen to disagree. On matters like piracy, which is certainly less detrimental to society than slavery was, the consequences for believing what is popular and what is right are relatively low. But to suggest that you can apply this principle across the board, in all situations, is, in my view, absurd.
You're suggesting there that we cannot judge anything that has happened in the past because our views on the matter are now different. But that means we cannot judge ANYTHING that has happened EVER because, by definition, now is different from then.
> You shouldn't be convicted that your morality is absolute and will always be the only correct one.
I'm not. I'm just not convinced that, because a certain activity is widespread, it is therefore morally correct.