Marriage has historically, as long as there’s been human history, meant a man and a woman in a relationship for life. Once we change that definition, then where does it go from there?
Well, I don’t think that’s a radical view to say we’re going to affirm marriage. I think the radical view is to say that we’re going to change the definition of marriage so that it can mean two men, two women, a man and three women, a man and a child, a man and animal. Again, once we change the definition, the door is open to change it again. I think the radical position is to make a change in what’s been historic.
This is a pretty clear example of the slippery slope argument. Just because we modify the definition of marriage to include two men or two women does not imply that we will then necessarily broaden the definition to include pedophilia or bestiality or polygamy (though, I should note the argument for polygamy is much more reasonable than for the other two). In fact, if one thinks a little bit about what marriage actually is, it will become clear that these two sorts of relationships would be impossible to legalize. In particular, it must be noted that marriage is primarily a contractual relationship. This means that in order for such a relationship to occur both parties must be able to enter into a contract. And this is simply not possible for an animal or a child.
In addition, the first part of this passage is a good example of the Ad Populum or Appeal to Tradition in that Huckabee is arguing that because marriage between a man and a woman is traditional it would be wrong to change it.