I don't need to make an argument for an absolute. Water is wet is an absolute. It is our description of what is.
Well, of course you have to make the argument! Just because something is absolute, doesn't mean that its self-evident. "Absolute" and "Self-evident" aren't synonyms. Even if they were, saying that something is self-evident does make it so either! Never treat any issue as though it's axiomatic unless and until you know with certainty that it is. Otherwise, someone could come around with something that shakes your whole worldview and you'll be caught flat footed. And, the absolute nature of morality is very definitely not axiomatic. You definitely want an argument at hand!
My business has gotten quite busy these last couple of weeks and so I don't have the extra time to make long posts as much as I did before. I'm completely out of time this morning but should have some time this afternoon or tomorrow. I'll present an argument that I think you'll appreciate and then the next time it comes up, you'll have some extra ammo.