Rick Hills has a series of excellent posts on slippery slopes (see here , here , and here ). He argues that slippery slope arguments should be regarded with suspicion, and he singles out the refrain from opponents of gay marriage: If you permit gay marriage, then you must also permit polygamous marriages, marriages between people and animals, and all other marriages between a person and any object of desire. Hills argues that such slippery slope arguments deny the existence of "conceptual ledges"—or, one might say, ethical ledges—that prevent the institution of marriage from sliding into the abyss of a sexual Babylon. We are capable of making distinctions on the basis of some moral theory about the purpose of an institution. If the purpose of marriage is to encourage long-term romantic relationships between two people, possibly though not necessarily with procreative consequences, then different-sex and same-sex marriage are morally indistinguishable. Why, Hills asks, does the slippery slope argument persist?
We can address Hills' puzzle about the popularity of slippery slope arguments by asking why no one made the following argument, say, 20 years ago when the idea of same-sex marriage was still as outrageous as the idea of polygamy is today. Why didn't, say, a feminist opponent of marriage make the following argument to conservatives who (mistakenly) believed that they should support traditional marriage: "You think you support traditional marriage. But if you define a traditional marriage as a long-term romantic relationship (etc., etc.) and you don't think that infertile and elderly people should be denied the right to marry, and thus you don't think it is essentially a procreative relationship, then you, via slippery slope, must also think that same-sex couples have the right to marry. Since you can't and don't think that, you should withdraw your support for opposite-sex marriage. People who make long-term commitments should receive no special legal status; otherwise, we are on the slippery slope to sexual Babylon."
Why would such an argument have seemed odd? The answer is that slippery slope arguments depend on a strong presumption in favor of the status quo, and thus are invoked only when someone advocates departing from the status quo (extending marriage rather than preserving it). This presumption in favor of the status quo reflects anxiety that if we depart from existing patterns of behavior, reason will not be strong enough to justify a new set of conventions that we can be happy with. Lawyers will recognize this anxiety from ordinary common-law reasoning. Judges rarely question precedents, and when lawyers argue that precedents should be overturned, the opposite side will make a slippery slope argument. The sanctity of precedent reflects the old cliché that it is better that the law be settled than be correct. The same point can be made about social conventions. That is why one sees both conservatives and liberals making slippery slope arguments. Liberals typically make these arguments when someone advocates relaxing civil liberties in order to address some new problem. "If we give the government the power to engage in surveillance of international calls without a warrant, we are on our way to dictatorship." So use of slippery slope arguments is not particularly conservative or liberal, at least not in a political sense. But it does reflect a conservative or legalistic habit of mind, one that fears departing from existing rules because of pessimism that society can arrive at a set of rules that are superior. Most social conventions are arbitrary—we could be equally happy or unhappy with a different set of conventions, and the particular moral theories used to justify existing conventions are frequently phony, after-the-fact rationalizations for the way things are—but some set of conventions is necessary for social life. So if we are constantly having to justify them, we will find that we can't, in which case we will incur very high transition costs for the sake of moving to another set of conventions that will, in short order, seem no less arbitrary than those from which we started.
The slippery slope argument has become shorthand for the view that if we try to make sure that all our social and legal conventions are morally justified, we will end up with no conventions or (what is almost the same thing) in a state of permanent transition between different conventions, which would be worse than tolerating existing conventions that are unjust. This argument is, at the extreme, an objection to any time of reform, and so cannot always carry the day, but it is not an argument that one can refute simply by showing that the proposed new convention is morally superior to the old one. But the reason this argument persists in the same-sex-marriage debate is that it is impossible to point out any concrete harms from recognizing same-sex marriage, and thus the only argument left is this anxiety about the arbitrariness of conventions.
Perhaps liberals and conservatives can make a pact: If liberals stop arguing that any reduction in civil liberties in order to combat terrorism will result in dictatorship, conservatives will stop arguing that any relaxation in our sexual conventions will lead to perdition. On the other hand, if both turn out to be right, we will find ourselves sliding in the direction of a police state that is a sexual Babylon—something for everyone, I guess. Odd that there is not a coalition yet that connects imperial flourishing with sexual and cultural as well as political decadence. We will have to await another Edward Gibbon to make this argument.