But disagreement doesn’t imply that there is no such thing as objective right and wrong, or rob us of any hope of moral consensus. We agree on the immorality of activities like murder or rape, so our disagreements over morality don’t imply that legislating based on morality is futile. Our cultural embrace of individually determined morality has just made it harder to reach agreement about what makes good law.
That insight, I think, leads us to the truth of the truism. “You can’t legislate morality” actually means “You shouldn’t make laws based on your morality, but on mine instead.”
Those who argue against legislating based on society’s current moral consensus, as expressed through its laws, are themselves making moral claims. The laws and the morality they’re based on are broken, they often say, because they restrain people from doing things that harm no one but themselves. Free expression is good, and harming others is bad. Laws that reflect these preferences are good laws.