In his Obergefell dissent, Chief Justice Roberts trots out a familiar slippery-slope argument:
[F]rom the standpoint of history and tradition, a leap from opposite-sex marriage to same-sex marriage is much greater than one from a two-person union to plural unions, which have deep roots in some cultures around the world. If the majority is willing to take the big leap, it is hard to see how it can say no to the shorter one.
Actually, it’s not that hard at all. For a detailed analysis of the constitutional problems with this argument, I recommend Stephen Macedo’s latest post at Slate. In terms of logic and public policy, however, we can answer Roberts with a short video, from my 2012 marriage series: