same-sex marriage and slippery slopes

The Masschusetts Supreme Court decision invalidating a ban on same-sex marriage is, obviously, a Good Thing.

What's troubling, besides the absurd "marriage is a sanctified institution and must be protected" (Religion, this is Government. Government, this is Religion. You two should stand in opposite corners of the room) argument is the slippery slope claim put forth, for example, by Eugene Volokh.

To be sure, he's put a lot more thought into slippery slopes than I have. But for him to suggest that the Massachusetts decision could lead directly to decisions legalizing polygamy and marriage between close relatives smacks of ingenuousness. Courts are entirely capable of judging each case on its own merits. Yes, precedence plays a key role in jurisprudence, but it's not everything.

This passage highlights a problem in Volokh's reasoning:

The court reasons that "the right to marry means little if it does not include the right to marry the person of one's choice," but while it qualifies this as "subject to appropriate government restrictions in the interests of public health, safety, and welfare," it's far from clear that a court would find that "health, safety, and welfare" would be hurt by adult polygamous marriages (assuming all existing partners in the marriage consent to the addition of another).

And yet, as Volokh himself points out, footnote 34 of the decision declares that "Nothing in our opinion today should be construed as relaxing or abrogating the consanguinity or polygamous prohibitions of our marriage laws." This footnote explicitly eliminates the slippery slope Volokh is talking about.

This isn't to say, of course, that a pro-polygamy or pro-incestuous marriage lobby couldn't bring forth arguments to extend the freedom of choice guaranteed to homosexual relationships in this decision. But those arguments would be evaluated on their own merits and failings. It doesn't appear at all evident to me that this decision necessarily provides the space in which those arguments would be persuasive.

There's a reason slippery slope arguments are considered fallacious, after all.