By turning marriage into a socially constructed reality that doesn’t have a nature, marriage can then be whatever you want it to be. Not just the union of a man and another man, but also even two men and a woman–three partners in marriage. Or it could be a man and a child. Or maybe even a man and his dog, if he feels close enough to his pet to want to marry it.(In context, William Lane Craig is arguing that long-term relationships are so uncommon among gay men, that they couldn't really be fighting for marriage for its own sake. Instead, their real goal must be to "deconstruct marriage". But I will ignore this context to focus on the slippery slope only.)
--William Lane Craig
I remember learning about logical fallacies in grade school English, and they would always include the "slippery slope fallacy" among others. I think this is wrong. A slippery slope argument is not necessarily fallacious. It depends on how it's used. Therefore, I call this a fallacious slippery slope argument to distinguish from those slippery slope arguments which are not fallacious.
First, I wish to divide slippery slope arguments into two categories (which are my own creation):
1. The slippery slope of reasoning
2. The slippery slope of consequences
Example of a slippery slope of reasoning: Suppose I claimed that all the best things are green. You could counter, "But if you follow that slippery slope, you must also believe that cats (which are not green) are not among the best things! Clearly this is absurd."
Another example: "If you believe that same-sex marriage should be legal, then you should also believe that man-dog marriage should be legal. This is clearly absurd."
This kind of slippery slope argument is no different from an argument ad absurdum. In a mathematical argument, we'd call it proof by contradiction. It's a logically valid argument, it's just that it's often unsound (ie the conclusions follow from the premises, it's just that the premises are wrong). In the green example, the person forgot to show that it is absurd to believe that cats are not the best things. In the example of same-sex marriage, the person forgot to show that if same-sex marriage is legal, then man-dog marriage should be legal. But if we granted the premises, the conclusions would follow.
William Lane Craig uses a slippery slope of consequences. He does not say that if we support same-sex marriage, we must logically also support man-dog marriage. Rather, he says that if same-sex marriage is legal, this would lead people to also legalize man-dog marriage in the future. The argument is not about what people should conclude from prior beliefs, it's about what people will do, how people will behave.
It's basically a moral argument. Lane Craig doesn't want people to marry dogs, so from his perspective it's worth taking actions to avoid this.
But there's something rather strange about this argument. We are free agents. We can either choose to allow man-dog marriage or not. We will choose according to our preferences. If we prefer to allow man-dog marriage, why should we prevent getting what we prefer? If we prefer not to allow man-dog marriage, then why would we choose to allow man-dog marriage?
Mind you, the slippery slope of consequences still isn't necessarily fallacious. There are some situations where we might worry that our future selves will be irrational. Or that other people will be irrational. Or that we'll have different preferences in the future. And there are game-theoretic situations where it's better to have fewer options (I've recently been reading about decision theory).
But none of those situations are relevant here. Making laws is a very deliberative process, not something that's decided on the spot on an irrational whim. And it's hard to imagine a non-trivial game-theoretic situation. Usually there are people who want a law, and people who don't, and that's that.
So despite the slippery slope argument being an acceptable argument in general, William Lane Craig manages to use a form that is completely fallacious.
On a related note, my boyfriend pointed out a slippery slope argument made by Supreme Court Justice Scalia, when he argued in favor of anti-sodomy laws in 2003:
"Today’s opinion dismantles the structure of constitutional law that has permitted a distinction to be made between heterosexual and homosexual unions, insofar as formal recognition in marriage is concerned. If moral disapprobation of homosexual conduct is 'no legitimate state interest' for purposes of proscribing that conduct; and if, as the Court coos (casting aside all pretense of neutrality), '[w]hen sexuality finds overt expression in intimate conduct with another person, the conduct can be but one element in a personal bond that is more enduring,' what justification could there possibly be for denying the benefits of marriage to homosexual couples exercising '[t]he liberty protected by the Constitution'?"I'm sure by now Scalia has thought of a few justifications for denying same-sex marriage that do not rely on anti-sodomy laws.
-Lawrence v. Texas, 539 U.S. 558, 605-06 (2003) (Scalia, J., dissenting) (internal citations omitted).