Same sex marriage and the "parade of tolerables"...

Rick's correct to note the general point on slippery slopes below, but I think his application of the point doesn't translate. Let me offer a couple quibbles re: the fine-grained legal theory. To get us to reach a conceptual ledge, Rick writes: Public nudity is, well, publicly visible; bestiality does not foster emotionally important human sociability; polygamy thrives on subordination of women; and so forth.

I recognize that Rick is being somewhat quick here, so I take it with a grain of salt, but consider:
a) public nudity may be no more shocking to us now than inter-racial coupling was to some communities in the past. And so the public aspect of public nudity is not really a conceptual ledge if what predicates opposition to it is merely aesthetic offense, no different in kind to the revulsion that racists had when they saw whites kissing blacks in public, or more recently, gays kissing each other in public.

Here the slope really is slippery--of course, its slipperiness is based on how pliable the social norms are. It might well be that the norms will be stickier here because of expectations of tangible harms. Maybe public nudity is opposed because it will cause rubbernecking and car accidents. But as society becomes more naked, less rubber-necking occurs. There's a dynamic loosening of the norm. In this vein, it's worth thinking of how some traditional societies view the allure of even fully-clothed women not wearing burkas or hijabs (or shaytels...). In those societies, if citizens wore what my students at FSU wear to class, it would be cause for commotion. Here and today, it barely registers.

b) the bestiality issue cannot rest on the ledge of failing to "foster" "emotionally important human sociability." First, that might suggest a reason not to have pets more generally. 

Second, it suggests that a person couldn't have sexual relations with a dog that are as emotionally meaningless as some of the hookups that occur in stripclubs, streetcorners, and bushes in the park. One might be a loving son and brother and husband, and still seek quick anonymous sex elsewhere. No necessary tension between human sociability capacities and quick sex with Trixie the sheep. (Think Woody Allen.)Bestiality to my mind is a concern because we can't think of animals as consenting meaningfully...  and it's the inherent coerciveness that seems problematic.  But if that logic does the work, it should also suggest a principle of vegetarianism because animals also don't consent to their slaughter.

c) polygamy is complex, as you know; much more so than just thriving on the subordination of women. One need only look at a) the existence of polyandry across time and space (which is admittedly less prevalent than polygny), and b) the fact that polygamy activists make their claims on several grounds including: i) it actually increases women's opportunities in the workplace vis-a-vis childcare responsibilities, and ii) it respects women's and men's liberty interests in polyamorous relations.

So only bestiality really rests upon a "conceptual ledge." The public nudity and polygamy examples are just aspects of social norms subject to the normative force of the liberty-advancing principles that supervened on miscegenation laws and now prohibitions against SSM. Let the slope slip against objections of "ickiness" -- at least as it relates to the use of the criminal law in a diverse liberal society.

Btw, there was a good post on Volokh.com recently by Dale Carpenter trying to draw the distinctions between SSM and polygamy.**  But color me unpersuaded. Dale links to this post by John Corvino, a philosopher interested in trying to stave off the slippery slope. It strikes me that Corvino's answer is missing the force of the liberty arguments at the heart of private ordering, and when SSM opponents adduce the parade of "horribles" there is no danger to those of us who believe it's really just a parade of "wonderfuls" or more plausibly, a parade of "tolerables."

**I can't link to it b/c for some reason the volokh.com site is not responding.