The slippery slope argument is often used to justify a certain legal position, whether involving legalizing an activity or banning it. For example some opponents of the Court's decision in Lawrence v. Texas argued that striking down a law prohibiting same-sex sodomy will lead to gay marriage and polygamy. Is such a rhetorical device truly a logical argument?
I've always believe that the slippery slope argument works well when dealing with people's conduct, but cannot successfully rebut an argument to change or pass a law. In Jewish law, many strictures were put into place banning permitted conduct because it could lead to illicit conduct. This is a form of the slippery slope argument. People might misconstrue the allowance for permissive conduct to be a signal to engage in impermissible conduct.
This argument makes sense. We cannot assume people are erudite in the nuances of the law. People will often fail to distinguish the legal bounds between legal and illegal. Hence we move the line to cover even permissible conduct to prevent people from sliding down the slippery slope to illicit conduct.
But in the case of making a law the argument does not stand. In short the argument is that if we legalize certain conduct, we will end up legalizing less appealing conduct. But the argument is flawed. Legislators and judges (who should not be making law, but that's a different topic) have the knowledge and time to dispassionately sit and analyze each law by itself. If they are convinced to legalize same-sex sodomy, it does not mean they will be convinced to legalize same-sex marriage. One step at a time.
Eugene Volokh lists a few circumstances where a law or decision was passed with the assurance that it would not serve as a precedent for other laws, but ended being just that precedent. For example, in Griswold v. Connecticut, a three member concurrence dismissed the slippery slope argument by reaffirming the state's right to regulate homosexuality. But in Lawrence, the Court used Griswold as the precedent to deny the state the right to do exactly that!
Volokh also makes some compelling arguments for the slippery slope argument. He distinguishes between types of slippery slopes. One major type is the cost-lowering slippery slope. By passing a law it makes it easier politically to pass a more distasteful law. His example is gun registration. Gun registration itself might not be a problem. But if guns are registered it becomes cheaper and easier to confiscate them. While confiscation without registration might be difficult to get done due to costs and the required political capital, once guns are registered it becomes easier to take them away.
Another type is the "attitude-altering slippery slope." Once something becomes illegal, people begin to assume its wrong. Immoral and illegal become easily conflated. So, for example, if the government outlawed assault weapons, people will start assuming, over time, that it's wrong to own those guns. It is a much shorter step to convincing people that outlawing all guns is a good thing.
He lists other types and the key conclusion offered by Volokh is that the slippery slope argument is not automatically illogical. But proponents must point out the mechanism under which the law will move down the slope.
A very solid argument from a pragmatic point of view. In theory, the government and people should be able to distinguish between the steps and decide on each one individually. In practice, that won't happen.
But despite what one thinks about the slippery slope in general, Volokh's arguments are food for thought.