Erastianism and Marriage in the Church of England

The Church of England's official prayer book and liturgy contains this commentary on and definition of marriage and its purposes, as written in the Book of Common Prayer:

Dearly beloved, we are gathered together here in the sight of God, and in the face of this Congregation, to join together this man and this woman in holy Matrimony; which is an honourable estate, instituted of God in the time of man's innocency, signifying unto us the mystical union that is betwixt Christ and his Church; which holy estate Christ adorned and beautified with his presence, and first miracle that he wrought, in Cana of Galilee; and is commended of Saint Paul to be honourable among all men: and therefore is not by any to be enterprised, nor taken in hand, unadvisedly, lightly, or wantonly, to satisfy men's carnal lusts and appetites, like brute beasts that have no understanding; but reverently, discreetly, advisedly, soberly, and in the fear of God; duly considering the causes for which Matrimony was ordained.

~First, It was ordained for the procreation of children, to be brought up in the fear and nurture of the Lord, and to the praise of his holy Name.

~Secondly, It was ordained for a remedy against sin, and to avoid fornication; that such persons as have not the gift of continency might marry, and keep themselves undefiled members of Christ's body.

~Thirdly, It was ordained for the mutual society, help, and comfort, that the one ought to have of the other, both in prosperity and adversity. Into which holy estate these two persons present come now to be joined. Therefore if any man can shew any just cause, why they may not lawfully be joined together, let him now speak, or else hereafter for ever hold his peace.

But now, as Father Dwight Longenecker states, the British Parliament has redefined marriage, superceding the authority of the Church of England's doctrine and order. This is the crucial issue for the authority of the Church of England--the reason Pusey, Newman, and Keble started writing Tracts and preaching sermons in Oxford. Whence its authority to teach as Christ's Church--from Jesus or from the State? Who is Head of the Church of England: Jesus or the monarch? Does the Church of England lead the State and the national culture or does the State and the national culture lead the Church of England? Are the teachings and discipline of the Church determined by divine or Erastian authority, based on revelation or on political trends? Those were exactly the issues that inspired the Oxford Movement. As John Henry Newman worked to substantiate the via media of the Anglican Church with apostolic and catholic authority, he found himself unable to find solid ground; thus his conversion to Catholicism in 1845.

Does Parliament subsequently to the passage of this law direct the Church of England to rewrite the Book of Common Prayer and this scriptural explanation and definition of Matrimony (marriage) cited above? Does English society achieve the parity and equality it desires if the Church of England has two different services: one according to the biblical faith of the Book of Common Prayer as currently published and one according to Parliamentary definition of marriage as a commitment? It's an Erastian knot that's impossible to untie--unless the official ties between the Church and the State are cut?