Same sex marriage does not undermine heterosexual marriage; all forms of committed relationship enhance and strengthen society.
The Church's formal rejection of gay marriage::-
* lacks the compassion of Jesus for all made in the image of God
* fosters an unjust and continuing discrimination (and in some cases abuse) of lgbt people in the name of Christ
*is in thrall to literal-minded understandings of scripture and tradition and lacks the energy and moral courage of Jesus who challenged unjust social structures and traditions
* further undermines the national credibility and relevance of the Church of England