If someone says morality comes from God you must ask a simple question: Is there a reason he made certain things right and certain things wrong, or does it merely depend on his own ruling? For example, if he were to say the rape and torture of children is ok, would that make it true?
If you say "yes", then slavery and polygamy is ok because it’s allowed in the Bible, and if you say "no"–because God would never say such things–then you can go directly to the reason he would not do so (which you already instinctively know) instead of relying on a God figure for morality at all.
[ My paraphrasing. ]