When a man and a woman get married and have children from another relationship, is it appropriate for the step children to date each other? I can't seem to get them to understand why that is inappropriate.

Its only inappropriate if their relationship is more of a family unit and they want to get together, because that's a little creepy. Family should not date and that's what you should tell them. Family is family and you love one another because you are family  but it should never be physical because its twisted (in my opinion). But if they feel its right then nothing can really stop them :P