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While the slippery slope may in fact exist, usually the argument is put forth while ignoring possible mitigating factors (such as informed consent for sex partners in the Santorum example), thus making it generally a somewhat-too-serious form of reductio ad absurdum.
 Parade of the Imaginary Horribles
The parade of the imaginary horribles is the list of off-the-wall worst-case scenarios that opponents claim will occur after the legalization or acceptance of a prohibited activity. (I.e., Santorum's "man-on-dog sex" statement listed above) These scenarios say more about the psyches of the people who devise them than about the proponents for change.
This term is attributed to former U.S. Supreme Court Justice William O. Douglas.
The slippery slope argument may have some validity when the matter under discussion is a proposed political compromise with an opponent who has an agenda with many more items on it besides the one on the table. This is usually called appeasement; the underlying argument is that agreeing to the proposed "compromise" will not buy you peace, but only leaves you in a weaker position when confronted with the next demand.
 Example of the slippery slope argument
Logical fallacies are a dangerous thing!
Avoid logical fallacies at all costs; there's no telling where they'll lead!
 Countering the slippery slope argument
Frequently, the slippery slope has no evidence presented for why one would actually go down the slope, even if it is hypothetically possible. Pointing this out is sufficient to logically counter the argument. However, slippery slope arguments that involve no evidence can usually also slip the other way: if it is argued that gay marriage will lead to man-on dog sex, one could just as well argue that restricting gay marriage will lead to banning marriage altogether. If evidence is presented for why the slip could occur, the argument may be valid, in which case countering it requires refuting the evidence.
 External Links
- "A Misconception About Slippery Slope Arguments" at The Volokh Conspiracy
- "The Mechanisms of the Slippery Slope" (PDF), Volokh again.