A theory explains a phenomenon by appeal to membership in a category.
Theories explain phenomena by appealing to some underlying cause or phenomena. Theories which do not appeal to an underlying cause, and instead simply appeal to membership in a category, commit the fallacy of limited depth.
My cat likes tuna because she's a cat.
This theory asserts only that cats like tuna, without explaining why cats like tuna. It thus does not explain why my cat likes tuna.
Ronald Reagan was militaristic because he was American.
True, he was American, but what was it about being American that made him militaristic? What caused him to act in this way? The theory does not tell us, and hence, does not offer a good explanation.
You're just saying that because you belong to the union.
This attempt at dismissal tries to explain your behaviour as frivolous. However, it fails because it is not an explanation at all. Suppose everyone in the union were to say that. Then what? We have to get deeper - we have to ask why they would say that - before we can decide that what they are saying is frivolous.
Theories of this sort attempt to explain a phenomenon by showing that it is part of a category of similar phenomenon. Accept this, then press for an explanation of the wider category of phenomenon. Argue that a theory refers to a cause, not a classification.