Caution--slippery slope ahead

From the increasingly incoherent Charles Krauthammer:

The "stirrings for the mainstreaming of polygamy (or, more accurately, polyamory) have their roots in the increasing legitimization of gay marriage. In an essay 10 years ago, I pointed out that it is utterly logical for polygamy rights to follow gay rights. After all, if traditional marriage is defined as the union of (1) two people of (2) opposite gender, and if, as gay marriage advocates insist, the gender requirement is nothing but prejudice, exclusion and an arbitrary denial of one's autonomous choices in love, then the first requirement--the number restriction (two and only two)--is a similarly arbitrary, discriminatory and indefensible denial of individual choice."

Good heavens, is there a better textbook demonstration of the logical flaw of the slippery slope? A rational adjustment to a legal definition must necessarily invite the wholesale eradication of the concept?

So, the opening of marriage to two consenting adults throws open the gate to polygamy, and what else Chuck? Cross-species marriage? Please.