Traditional marriage relied on the inequality of gender. The woman took the man's name, wears a veil at the ceremony (related to Middle Eastern female coverings), and is given away as property from father to groom. Marriage typically lasted a lifetime because a woman could not survive on her own. She was blocked out of careers and most ways of earning a living. Because of her reduced version of personhood, she could remain successfully subject to her husband, and so marriage would last a lifetime. Beautiful, traditional marriage in which one human dominates the personhood of another. This had created the bulwark of how society at large functioned with the hierarchies of power and relative levels of personhood. The great Patriarchy.
Gay marriage undermines these hierarchical and exploitive structures in society. It overturns some of our deepest immoralities. This is why it has my vote.