As ridiculous as the following may sound, it is absolutely true. In the legal sense in America, the institution of marriage is not based upon love or commitment, or the desire to become a union or create a family. It is not based upon our religious beliefs. The "qualification" for marriage is based entirely upon our anatomies.
In the eyes of the government, two men or two women cannot recite the vows of marriage. If one of the men and one of the women were to change their anatomy to the opposite, then all criteria would suddenly be met. This would make a man and woman who could enter into matrimony, and suddenly they could become a family respected as any other in the United States. Inside they would remain the same, but outside they would "fit" society. Neither the government nor the people could disallow the couple to become as one.