I had allowed the taping of a same-sex marriage in Trinity Church, San Francisco where I am rector. The outward and visible signs in marriage are two people. The inward and spiritual grace is the couple's vows and the assurance of God's blessing on the couple. Marriage conveys what it signifies.
Marriage conveys vows of fidelity, life long union and love.
One doesn't have to be of the opposite sex to convey the significance of marriage. We also know that the ministers in the marriage are not the clergy, but the couple. This means that the sacrament of marriage happens with or without the clergy and the church. It happens when the couple chooses to enter into the covenant of marriage. They may go to the church and ask the assistance of the clergy for counsel, prayer and, in the American church, sign some legal documents.