I like to say that the slippery slope argument is a request for a limiting principle. Take getting rid of Sodomy laws as an example.

"If we let them have sex with each other, what's going to stop them from trying to get married? And then what if they start wanting to marry teenagers to older men? And then kids?"

Reasonable libertarian: the government shouldn't be involved in discussions of marriage, nor in the sexual choices of consenting adults. Your church shouldn't be forced to acknowledge someone's claim of being married, and marriage is a religious ceremony that shouldn't even be registered with the government, let alone carry benefits. Anyone trying to involve children in sexual matters is a pedophile who deserves exile at a bare minimum. Teenagers should only be experimenting ssxually with other teenagers of a similar maturity. Anything else, pedo shit that deserves exile at a bare minimum. All of these are limiting principles.

Pedo progressive: "We aren't asking for that (mutters: yet).

And that's how you tell the difference.