Why Can Two Sincere People Still Disagree Morally?

Two people may understand the situation, speak honestly and still reach opposite moral judgements about the same act. This is not mysterious. Sincerity means that each person states their judgement truthfully; it does not ensure that they share the same account of the facts, ranking of values or standards of reasoning.

Contemporary debate often returns to John Mackie's “argument from relativity”. Deep and persistent variation between societies may seem easier to explain through different ways of life and moral frameworks than through their grasp of one set of objective moral facts. Moral realists reply that disagreement alone does not show that there is no fact of the matter: science also contains sincere, enduring disputes. Some apparently moral conflicts may also rest on different beliefs about consequences, circumstances or another person's intentions.

My view is that sincere disagreement should reduce our confidence that our own position is obviously right, but it need not turn morality into mere taste. In practice, first ask whether the parties agree about the facts. Then ask whether freedom, fairness, harm or other values are being ranked differently. Only after that should we decide whether their standards genuinely resist translation. Sincerity is not the answer; it is what makes the disagreement worth analysing carefully.

https://plato.stanford.edu/entries/disagreement-moral/
https://plato.stanford.edu/entries/moral-relativism/