Aumann agreement theorem

You're getting into apples and oranges here. Unless you're going to pull a Sam Harris and argue that moral propositions are "facts" like physical law.

No, I didn't suggest you can treat moral judgements as "facts" - although you can demonstrate their consequences in a factual manner ("Gay marriage will lead to bestiality!!" - "Well, just watch it not do that."). But demonstrating something to be factually inaccurate, or demonstrating an ethical position to be badly thought out is fairly trivial next to convincing the believer that it's so.