Back in the days when the Church of England did not ordain men who had remarried whilst a former spouse was still alive it still accepted that couples legally married outside of the church were really married. They had to as they are an established church and as such could hardly claim that the secular legal system was not effective in such matters. Mrs MP and myself were married in a registry office and when I was being encouraged to seek ordination there were concerns about my divorce from my first wife but nothing was ever said about the fact that my second wedding did not take place in a church. We were not told that we had to go through it all again in church after General Synod voted to allow divorced people to marry in church.
So the question is this, will the Church of England recognise that a same sex couple who marry in an English registry office are really married? If yes, then why not marry such couples in church? If no, will the Church of England be claiming that they are no longer, in practice, an established church?