Marriage in the US is a CIVIL institution. It may have originated as a religious ceremony, but the religious aspect MUST be ignored in regards to considering legality. In the American model, God(s) only enters into it when both people want Him(Them, It) to, hence the authority of a Justice of the Peace to perform a marriage. When a pastor performs a marriage 'in the sight of God', there are certain civil requirements he or she must perform in order for the marriage to be recognized by the state. As far as the law is concerned, God does not unite the two people, the state does.