I believe your argument strategy can be understood without thinking about analogies at all. From your example, the person claimed the strong statement of the form "the only thing that satisfies property X is Y" (namely that the only marriage that is valid is christian marriage)¹. You were simply testing the person if they really believe in that strong statement, by checking other things beyond Y that perhaps could also have property X, in which case the person would be forced to admit that their original premise is flawed, forcing them to look for another argument. Looks OK to me.

Just don't forget that this is a strategy to counter the person's argument, but doesn't mean you're right either.

¹ I used the term "strong statement" because it is a statement that claim something about many things (namely every type of marriage, claiming that it is either christian or invalid).