Marriage confers very special rights, such as permanent residency or social security inheritance, that you cannot obtain through private contracts. Therefore the government has to be involved.
this is the crux of the issue in that government policy is, in large part, based upon a private contract. neither government nor judeo-christian sects (which, if we're honest, is whom we have in view when we speak of "religion") seem to be able to grasp the full implications of this.
