Slippery judicial slopes

The Democracy in America blog at The Economist has a post which is relatively sympathetic to the argument that the American voters should decide when they are ready for gay marriage, not the Supreme Court.  In it, they note the slippery slope argument for polygamy as follows:

The states' arguments taste of rather weak tea, but the line-drawing point should give pause to even the liberal justices. In framing their view as one of "marriage equality", and in urging a shift from procreation to state-recognised intimacy as the basis of marriage, the challengers of the state bans open themselves up to worries about where this all ends. We don't have to revert to Rick Santorum's ridiculous comparison of homosexuality to "man on child, man on dog, or whatever the case may be" to imagine polygamists pressing for their day in court were the justices to affirm a new constitutional right to same-sex marriage. There may be much better policy arguments for a ban on polygamy than for prohibitions on gay marriage (and there certainly are strong state interests in maintaining age-of-consent laws) so the worry is not that handing gays and lesbians this new right will destroy marriage as we know it. But the issue will come up, and the justices need to find a way to expand the boundaries of marriage without erasing them.