"No authority – civil or religious – has the power to modify the fundamental nature of marriage."

Below is a statement from Archbishop Peter Smith, lately of Cardiff, now of Southwark
There would have been a time when Anglicans and Roman Catholics would have been in complete agreement over this. Some maintain that officially we still are. However, never underestimate the ability of  powerful lobby groups within the Anglican world to subvert and then shift orthodox positions. For a related example see here
Note: again we see the essential limit to the authority of the Catholic Church - a humble recognition that there are certain "givens" of the divine and the natural law which cannot simply be changed to suit the spirit of the times. In stark contrast, underlying much of contemporary liberal revisionist theology seems to be a reckless antinomianism.

"The Government statement on 17th February makes it clear that they are now considering a fundamental change to the status of marriage. That is something which was never envisaged by the Equality Act or any other legislation passed by Parliament. Marriage does not belong to the State any more than it belongs to the Church. It is a fundamental human institution rooted in human nature itself. It is a lifelong commitment of a man and a woman to each other, publicly entered into, for their mutual well-being and for the procreation and upbringing of children. No authority – civil or religious – has the power to modify the fundamental nature of marriage. We will be opposing such a change in the strongest terms.

The Equality Act was amended to permit Civil Partnerships on religious premises, which unhelpfully blurs the distinction previously upheld by Parliament and the Courts between marriage and civil partnerships. A consenting Minister is perfectly free to hold a religious ceremony either before or after a Civil Partnership. That is a matter of religious freedom, but it requires no legislation by the State. We do not believe it is either necessary or desirable to allow the registration of civil partnerships on religious premises. These will not take place in Catholic churches."