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Or consider another restriction built into the traditional definition of marriage: that the married couple be unrelated to each other. The Kings and Queens of Europe defied this taboo, merrily marrying their cousins, with tragic genetic consequences for their offspring. For gay marriage there are no such genetic consequences. The child of a gay couple would either be adopted or the biological product of only one parent. Therefore the fundamental basis for the incest taboo disappears in gay marriage.
Do gay-marriage advocates propose to permit the marriage of, say, two brothers, or of a mother and her (adult) daughter? If not, by what reason of logic or morality?
The problem here is not the slippery slope. It is not that if society allows gay marriage, society will then allow polygamy or incest. It won't. The people won't allow polygamy or incest. Even the gay-marriage advocates won't allow it.
The point is why they won't allow it. They won't allow it because they think polygamy and incest wrong or unnatural or perhaps harmful. At bottom, because they find these practices psychologically or morally abhorrent, certainly undeserving of society's blessing.
Well, that is how most Americans feel about homosexual marriage, which constitutes the ultimate societal declaration of the moral equality of homosexuality and heterosexuality. They don't feel that way, and they don't want society to say so. They don't want their schools, for example, to teach their daughters that society is entirely indifferent whether they marry a woman or a man. Given the choice between what Sullivan calls the virtually normal (homosexuality) and the normal, they choose for themselves, and hope for their children, the normal.
They do so because of various considerations: tradition, utility, religion, moral preference. Not good enough reasons, say the gay activists. No? Then show me yours for opposing polygamy and incest.