Blackstone defines a Law as anything that is not socially acceptable. God's Laws are one written in the Bible. Also you will find it in the Bible that Man's laws are God's laws too. I think that is in Romans, around Chap 13 verse 5 I believe.

Now like you say I'm not going to take my daughter out and stone her to death for crawling in the backseat with Little Johnny, but according to Jesus Christ those kind of laws are paid for by him.

So If the Bible says slavery is ok, more than half the Country says the slavery is ok. Where does anyone come up with "well their wrong"? Our laws and moralities are built upon what society deems proper.