Opponents of Gay Marriage often whip out the following argument: "If we say it's all right for two men to marry then why not three men? Or two men and one woman? Or fifteen people?" That always strikes me as one of those "we can't make a good case so let's make a stupid one" arguments.
Most "slippery slope" arguments are like that. My neighbor has a dog. I don't like dogs but I can't argue for a dog ban based on that so I try slippery-sloping it: "If we let anyone who wants a dog have one then what's to stop my neighbor from having a rabid coyote? Or fifty disease-carrying pumas that will kill our children?" Back when there was a debate in this country about lowering the voting age from 21 to 18, all some opponents could come up with was "If we lower the age to 18 then why not to 17? Then 16-year-olds will demand the vote, then 15 and before you know it, we'll have three-year-olds voting! Then embryos!"
There are, I will concede, some legit arguments of this nature…cases where doing X is likely to lead to Y. But most of the time, the principle is that if you can't gin up a reality-based thesis, you invent one based on something that's not apt to follow.
Jay Michaelson discusses the one where they leap from Same-Sex Wedlock to polygamy…or as he calls it, polyamory. He thinks we don't know enough about multiple-partner relationships to say if they're at all in the same category as same-sex ones. I think it doesn't matter much until such time as there's a real outcry to consider legalizing group marriages.