This post by Sam Harris gets at something that I suspect is at the crux of much of the disagreement over harm reduction. He argues that there are objectively true answers to moral questions.
There is undoubtedly a great deal of truth in what he says, but the potential for excessive certitude is troubling. It feels like it creates more ground for people to indulge moral vanity rather than less. He introduces a contrast between there being no answers in practice vs, no answers in principle, but it point is to emphasize that there is a correct answer.
He seems to define the correct answer to moral problems as the answer that contributes most to human happiness and thriving–so, he appears to be arguing for a form of utilitarianism, probably a form of rule utilitarianism. It seems, to me, that you can’t avoid subjective judgement when we get into defining happiness and thriving and what conditions or acts create happiness and thriving, however we choose to define them.