I'm not sure of the point you're getting at. You can do this with effectively anything. Replace "gay marriage" with "child sexual exploitation" in "I support gay marriage". Does that mean we shouldn't support anything?
I understand you wanted to refute the former comment, but, ironically, that is indeed the case in point: when we cannot trust that the the thing we support won't be exchanged with something else in the future we don't support, without our knowledge, then we indeed should not support it.
