You can’t hate the slippery slope fallacy out of existence; it’s real, it applies here, and it means the argument being put forward, that allowing one thing must mean we allow something similar but worse, is not a valid one.
It’s not magic, it’s simply rational. Allowing something doesn’t mean you must then allow something else, that isn’t how it works, and it’s probably more noxious and damaging to society that such a thought is allowed to propagate such as what you’ve written here.
There's a reason we have the phrase "crossing the Rubicon". You can sneer and say that crossing a river doesn't have anything to do with the fall of the Roman Republic all you like.
It’s not magic, it’s simply rational. Allowing something doesn’t mean you must then allow something else, that isn’t how it works, and it’s probably more noxious and damaging to society that such a thought is allowed to propagate such as what you’ve written here.