This has absolutely nothing to do with same-sex marriage!
This is designed to evoke a predetermined response. It seems that every time someone claims, regarding marriage equality, that children do best with a mommy and a daddy, we indulge them with a discourse on the fundamental soundness of gays as parents. It's irrelevant.
Regardless of the status of marriage equality, the same heterosexual couples are going to unite in the same marriages, crank out the same children and sue for the same divorces. Whether or not gays are permitted to marry does not affect who raises a single child.
Now if people are concerned about kids, it is a fact of life that gays are raising about two million children in the United States. Many of those parents are in committed unions and their children deserve the stability and security of having married parents.