This isn't going to be a popular answer, but:
Why not stop refuting it? The slippery slope exists for a reason – to bring rights, opportunities, and a chance at a fair life to an ever-widening group of people.
In the case of marriage, it's clear that there is a religious and political entanglement issue. One of the BEST ways to resolve this would be to provide clarity and simplicity in the LAW, that allows for easy understanding by all, rather than a lot of exceptions and addenda tacked on to a clearly flawed starting point.
To that end, I would propose two distinct de-couplings, long overdue:
- religious marriage from the law, completely. No, you can't have Gay Marriage if Allah doesn't want Gay Marriage. Go fight with your Mullah about that, please. Civil unions for all under the law, without any regulation of or restriction to the laws of marriage within a given religion. There simply is no need for the two to agree/be consistent with each other. At least, not if we tidy up a few outstanding next-of-kin/power of attorney laws to reflect this equality/neutrality, rather than going after the "easy fix" of adding additional supported classes using the existing wording.
- legal responsibilities and powers from sexual intercourse. Seriously, we could solve any potential future marriage and the law issue by boiling it down to this: non-genetically-linked succession rights. What does putting a wee-wee into a hoo-hoo versus a no-no have to do with it? It's downright childish, I tell you… Which reminds me…
We already have much better laws on the books to handle this: it's called adoption. Why not simply let any adult adopt any other adult, or even multiple other adults? In fact, why not allow an option to allow for both "mutual and asyncronous adoption", in which such rights could be conferred one-way? This would cover gay marriage, polyamorous marriage, all kinds of heretofore unrecognizable pair-bond dynamics in longeterm BDSM and even vanilla extra-marital affairs! Go, unexploited tax base, go!
Simply put, it's not the "slippery slope" that is the problem, it's that people on every side of the problem are looking at "tunnel vision" solutions, that either "preserve sanctity" or "confirm validity" of a single interest group, which only serves to further highlight/dissociate/exclude such groups from the whole – for every good achieved using this method, it seems bound to create new enemies. If we can make such arguments about "why people are bigots who shouldn't be listened to", rather than about "how group X is actually kinda normal, kinda", I'd argue that it would be a hell of a lot easier to stamp out such bigotry in a less divisive manner.
How can supporters of gay marriage refute "slippery slope" arguments?