What Slippery Slope?

"Legalize Polygamy!" That's the headline of a piece in Slate magazine. Okay, let's hear why allowing gay marriage on the bases put forth won't lead to this and who knows what else. But we love each other - why can't I marry my __________*?

* sister, mother, dog, Packy the Pachyderm, bass boat,  etc., etc., etc.
www.slate.com/articles/double_x/doublex/2013/04/legalize_polygamy_marriage_equality_for_all.html