What I do not like in this picture is that it influences people to make two wrong assumptions: 1. to think that law is always (or most of time) protecting immoral things and
2. that things that are illegal now might become legal later (independent of morality).
There are another points that IMHO shall be clearly noted:
1. At some point even the most terrible things could be accepted by public as morally normal and fixed in the law.
2. Public morale changes, law follows.
Well, public morale is clearly impacting selection of those who has the power. Public morale is a basis that allows for selection of powerful people. If it is acceptable for majority to have slaves than this majority will elect representatives that will make slavery legally regulated and convenient.