I use that headline to make the point of what a faux argument "marriage equality" is. That's NOT what proponents of gay marriage support. They support extending marriage to gays, period. That's not equality. Society has become generally accepting of gays and therefore they support extending marriage to them. Gays don't even support marriage equality. They bristle at the notion that polygamy is as morally valid as homosexuality. They still believe in a fence around marriage; they simply want to be on the inside looking out.
The reality is that there is a larger polygamy movement in America than people realize and is driven by a more diverse contingency than many may imagine. Had SCOTUS recognized a constitutional right to marry, you would have seen it surface very quickly. I suspect SCOTUS may have had that in mind as it considered a pair of cases seen as "gay marriage" cases.
Proponents of gay marriage see no moral superiority to straight marriage over gay marriage. But they see a moral superiority to straight and gay marriage of polygamous marriage. Ask them to explain that. It's the same blank look you'll get when you ask them to explain how Carbon Dioxide causes the planet to warm. I don't support polygamous marriage either. But I know hypocrisy when I see it. And they can call this a fallacious slippery slope argument...until gay marriage is legal almost everywhere and polygamists coming looking for "marriage equality." And they will.
Unrelated exit question: remember when opponents of the 2006 amendment argued it wasn't needed because gay marriage was already illegal in Wisconsin? In fact, they ran deceptive ads suggesting that if you opposed gay marriage, you should vote no on the amendment, "to keep gay marriage illegal, like it is now." However you feel about gay marriage where we are now shows just how fraudulent that campaign was.