Kids Need Both Parents

The title of this article is a very common Father's Supremacist group (and affiliates) statement. I feel the need to ask,

Why do kids need both parents?

And when they say this, they mean both natural born parents. What makes them (natural borns) so special?

What's wrong with just a mom? Or a mom and another man? Or hell, two moms? Or a just a grandma?

On the flip side what's wrong with just a dad, or a dad and another woman, or two dads, or a granddad?

How do kids need both parents? What does it look like when you need both parents? Could you pick a kid out of a crowd and determine that he/she needs both parents? If a kid doesn't have both parents, what does this reveal about him/her? How does having both parents fix many or all of the problems that plague kids?

I am looking for a serious answer here. I am willing to entertain whatever thoughts you may have on the subject.