Rick Hills roundly condemns slippery slope arguments:
The California Supreme Court’s recent decision on gay marriage has predictably revived that old perennial favorite of arguments against substantive due process arguments for sexual privacy — the “slippery slope.”
You know the drill: If courts strike down x law regulating sexual conduct, then it will be logically impossible to avoid striking down y, z, a, b, and c laws…. The obvious response to the “slippery slope” is the “conceptual ledge”: There are lots of natural resting places for the mind, if one only bothers to look for some fine-grained moral/legal theory…. So why do patently unconvincing slippery slope arguments grow like black mold in a leaky attic every time a court makes a decision about sexuality? …
It’s true that arguments that it will be logically impossible to distinguish a future case from this case are usually very weak. Very few distinctions are logically impossible.
But slippery slope arguments endure partly because they are often cast, much more plausibly, as arguments that if X is done, it will become more likely that Y will be done — not that X and Y can’t be treated differently, but that they won’t be treated differently. These arguments aren’t about logical consequences, but about psychological consequences (plus some other consequences). And as such they can’t be rebutted simply by pointing out that a distinction could be drawn.
The other reason that slippery slope arguments endure is that slippery slopes do often seem visible. I say “seem” because it’s often impossible to tell for sure whether X increased the likelihood of Y, or whether Y would have happened in any event. But sometimes there’s good reason to think that slippage has happened, often despite the express insistence of backers of X that of course X won’t help lead to Y.
I discuss this in detail in my Same-Sex Marriage and Slippery Slopes. I start by pointing to two examples:
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The slippage from contraceptive rights for married couples (Griswold) to contraceptive rights for unmarried couples (Eisenstadt) to abortion rights (Roe) to sexual autonomy rights (Lawrence), which happened despite the express assurance of some backers of the first step that it wouldn’t lead to later steps.
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The reliance of the Massachusetts and California same-sex marriage decision and the Vermont same-sex civil union decision on the enactment of other gay-rights laws, which happened despite express assurances of some backers of the earlier steps that they wouldn’t lead to the later step.
I also explain how some specific arguments for same-sex marriage, if accepted, can indeed weaken public, legislative, and judicial resistance to calls for polygamous marriage — for instance, “all people have a right to marry whomever they choose,” “it’s none of my business whom someone else marries,” “people who want to enter into same-sex marriages should have equal rights with those who want to enter into opposite-sex marriages,” or “love should prevail over arbitrary legal restrictions” (see the article for citations).
This having been said, I conclude that it seems unlikely that accepting same-sex marriages will materially increase the chance that society will accept polygamous marriages. But that kind of decision has to be tied to a pretty careful analysis of the likely psychological and political processes that are likely to take place. It can’t be based on a simple categorical refusal to consider slippery slope arguments whenever a logical distinction is available, or a simple categorical acceptance of slippery slope arguments whenever a similarity can be logically pointed to.