No, it's not inherently a fallacy. Case in point, the Patriot Act and everything that followed.
Yes, it can be used to support idiotic arguments like that legalizing gay marriage will lead to beastiality, or anything that Megan McArdle will use it to support, but it shouldn't be automatically dismissed as an invalid concern.
I genuinely can't tell if you're an elaborate troll, LLM, or just autistic.
I don't mean any of those as an insult, just to say that you have a very distinct prose that really comes through when you write more than a sentence, or two.
Given your post history, I'm leaning towards LLM, but I'm open to being wrong.