Marriage is not an institution because of the opposite sexes, it's an institution because of love. To give a quick antidote, Mildred and Richard Loving, the "Loving" from Loving v. Virginia, brought about massive change in the idea of marriage in the United States. They were part of an interracial marriage, one of the first (granted, of opposite sex). In order to be a legally married couple they had to sue the state of Virginia, and appeal to the point where it went to the Supreme Court. This is a extremely stressful, expensive, and prolonged undertaking. Now, if marriage was based solely on the need for the opposite sex, both of them (Mildred and Richard) would have left the other to find a different people of the same race. But they didn't leave each other. Both of them knew the other was the one person they loved, and no one else could replace that love. Shortly after Mildred's death in 2008, their daughter made the statement that "I want (people) to remember her as being strong and brave, yet humble — and believed in love." 

It was love that brought them together. It was love at bund them to one another. It was not solely their opposing sexes that promoted these two people to change the face and function of marriage in the United States. It was their love for one another. It was love that made stand up and face the persecution that would be placed upon them. It was love that made them stand up to injustice and say "No more!" It was love that made them stand for love. It was love that brought them together, and it was love that kept them together. And after Richard's untimely death, it was love that kept him alive in her memories, and it was love that gave her the strength to move forward. For those who say interracial marriage and same sex marriage are not comparable, are undoubtedly mistaken. They are both united through love, like the rest of us.