The current marriage laws are an odd mixture, and there are various forms of marriages or ceremonies which can take place. With a registrar present in a place of worship, without a registrar present in a place of worship. at a registry office as a civil marriage or as a "civil partnership".
The currrent government talks about trimming down red tape, you could trim down a lot of red tape by simply calling all those civil partnerships which have taken place so far "civil marriages", disestablishing the civil partnership ceremonies and allowing same sex couples to enjoy a civil marriage ceremony.
Additionally, why the necessity to have registrars attending some religious ceremonies but not others? If they aren't necessary at one institution, they aren't necessary at another.
This is important as separate but equal is not equal.