There was a letter in the Guardian this morning from the three denominations seeking this change: Church partnerships.
Richard Harries has written an article, now available at Cif belief Commitment we should encourage. Here is part of what he says:
…Some Church of England bishops, who were hardly enthusiastic about civil partnerships in the first place, fear that if this is allowed it would blur the distinction [between] them and marriage. But this is a fundamental issue of religious freedom. On what grounds can any body claim religious freedom for itself but deny it to others? The bishops may or may not approve of what Quakers, Liberal Jews and Unitarians want, but that is beside the point. What these bodies want would harm no one, and it accords with their deepest religious convictions. Religious freedom is indivisible. The only reason for denying it must be that of John Stuart Mill, namely if some public harm would result.
The harm to be taken into account need not be only physical, as race relations legislation shows. So it could be argued that allowing some faith communities to perform civil ceremonies on their premises was harmful in the sense that it undermines the institution of marriage in our society. But just the opposite is true. If we accept the argument that we need to retain both the term marriage and the term civil partnership, and that they are not identical, it seems to me clear from a Christian point of view that a ceremony in which two people commit themselves to a faithful, lifelong relationship before witnesses, partakes of the nature of a marriage. As such, from a Christian point of view, it can also express the biblical truth that such a relationship reflects the undeviating faithfulness of God towards us and which, according to St Paul, has its prototype in the relationship of christ to his church…