It is not all preference based, as very few moral systems lead to thriving societies that are stable and able to self-reproduce rather than collapsing back into chaos. In fact, almost none do.
Take a look at the ten commandments. "Honor thy father and mother so that it may be well with you and you may live long in the land". What happens when generation n+1 thinks they are morally superior to generation n? The same also holds for n+2, and so you do not have a stable society, you have a disintegrating society. Or half the commandments banning envy. Why ban envy and wanting what your neighbor has? Because envy-based moral systems lead to less successful societies than charity based moral systems, where the rich are told they should be generous to the poor versus telling the poor that they have a right to something possessed by the rich. It is not arbitrary -- some ethical codes lead to stable, successful societies and others do not.
One of the problems with modernism is that we have thrown away this notion of reverence and replaced it with a belief in moral progress, which has only led to an ocean of murders and social disintegration. In 10 generations, there will be no modernism left, it will be re-absorbed into more traditional societies, because modernist societies aren't able to reproduce themselves. Not biologically, not ethically, not economically. Yes, there are a few Gene Roddenberry holdouts that believe in generic liberalism as a system on which a society can be based. But the voice of LaFayette is always drowned out by the voice of Robespierre. Lafayette was a fool, thinking that whatever seemed right to him could be the basis of a society.
So it's not so much that I believe I can win an argument with someone who believes moral systems are preference based, but that those who do believe that will simply be outcompeted. Societies in which large numbers hold to such views will be unable to reproduce themselves and they will be eclipsed by societies that adopt ethical codes that form specific templates. A painter can paint a picture of any creature, but actual living, successful organisms are under strict constraints. A philosopher can imagine any moral code, but living, successful societies have to stick to what actually works.