Directly and indirectly via Jane Galt, I've come across two articles making the "slippery slope" argument against gay marriage. The argument is essentially that if we legalize gay marriage, then we'll soon be allowing polygamy, bestiality, pedophilia, and whatever other types of undesirable relationships the author can think of. Because these more unconventional relationships are either undesirable in and of themselves, or because (as Stanley Kurtz argues in the second link) allowing so many relationships would make marriage meaningless, we don't dare loosen the definition of marriage to include same-sex couples.

What I find interesting about this argument is that it implies that there's nothing inherently wrong with gay marriage, and nothing particularly special about heterosexual monogamy. The only thing wrong with gay marriage is that legalizing it would be a change, and the only thing that heterosexual monogamy has going for it is that it's the current status quo. This is a conservative argument in the classical sense. (Conservatism as I'm using it here is not synonymous with right-wing philosophies -- for example, the argument that homosexuality is a sin is a moralist argument used by the right. Classical conservatism can be found on the left as well, for example in the "precautionary principle" with regard to the environment. I should also point out that this definition, while applicable to the case at hand, is more strict than the definition offered by John Quiggin, who characterizes conservatism as a preference for gradual or bottom-up change over sudden or imposed change.) Classical conservatism is risk-averse with respect to new practices. It assumes that, in the absence of substantial proof, changing society is too risky and brings with it too many potential catastrophes to be desirable.

In the case of gay marriage, one could make an equivalent slippery slope argument against a (hypothetical) tightening of the definition of marriage. If we outlaw interracial marriage, it opens the door to outlawing marriages between the rich and poor, and between people whose parents don't approve, and pretty soon nobody will be able to meet the requirements for getting married. When we put the two slippery slopes together, what we get is a view of society precariously balanced on the status quo. A little movement could send things spiralling off into destruction. In this way, conservatism offers little hope for the future, as it postulates that the best we can do is to stay where we are (or perhaps backtrack a bit on the most recent social changes). This involves the assumption that social stability is achievable (doubtful), present in the status quo (wrong), and desirable (questionable, depending on the context).

I don't mean to suggest that slippery slope arguments are always bad, though I don't find the one about gay marriage particularly convincing. But I am skeptical about a philosophy whose basis is slippery slopes.