End of issue. Because it suggests that once society has spoken, in this case speaking through the law, that that's the final word. There's nothing else to say about it. That's the highest word on morality. In other words, there is no law above society.
Moral rules are relative to society and once society has spoken, end of issue.
The discussion is over. You might already be thinking of some of the problems with that way of thinking. First of all, it makes it impossible to criticize another society's practices. One could well use that same objection to other's objections to apartheid in South Africa. We could say, well, that is their society.