While Stanley Kurtz claims he has won the slippery slope debate outright, his analysis, here, is reasonably limited to the dangers of polygamy and polyamory. But beyond just the policy differences between the two, there is also a legal bulwark between Justice Kennedy's reasoning in Lawrence v. Texas (and the Massachusetts decision in Goodridge v. Department of Public Health, which borrowed heavily from the reasoning of Lawrence) and the invasion of the polygamists: The right to sexual privacy Kennedy finds in the line of cases starting with Griswold v. Connecticut, the Connecticut birth-control case from 1965, is an intimate right, between two consenting partners. The court calls these "the most intimate and personal choices a person may make in a lifetime, choices central to personal dignity and autonomy." The desire of a group of seven people to marry simply does not intuitively fit into that binary sphere of intimacy.
Just because advocates of polygamy have tried to leverage the Lawrencedecision to support their cause doesn't mean there are no differences between the two marginalized groups. And it's just not an argument against gay marriage to say, "I told you those bigamists would use this in court!" It would be stupid for the bigamists not to try.
One of the most persistent complaints of conservative commentators is that liberal activist judges refuse to decide the case before them and instead use the law to reshape the entire legal landscape for years to come. The Massachusetts Supreme Court, in finding that the ban on gay marriage violated the state constitution, did exactly what good judges ought to do: It confined its reasoning to the case before it, rather than addressing the myriad hypothetical future cases that may be affected by the decision. Opponents of gay marriage should consider doing the same.