Because it's true equality. That's the only reason.
If same-sex marriage was simply referred to as same-sex marriage I'd have no problem with it. I'm not a polygamist and have no desire to push for polygamy.
Unfortunately, it's same-sex marriage is not referred to the way it should be. "Marriage equality" is a false term used as a Trojan Horse to hijack morality and guilt people into adhering to the progressive agenda. It's a standard trick, used in others areas such as the term "carbon pollution" (when carbon dioxide is not a pollutant).
True marriage equality would include polygamy. That's a fact.
***
The thing with Gronk is, he doesn't actually think for himself. He just parrots what the brainless leftoid hordes parrot so that he fits in and can think of himself as a morally superior progressive.
His comments are irrelevant.