This is a fairly erudite book from a good scholar, who has much of his life invested in the issues discussed. Much of the theological groundwork has been updated by various documents and articles emerging from the 2008 Lambeth Conference.
Dr John makes his point clearly, with a pragmatic method consistent with modern Angican thinking. It is pragmatism at its best, and many will be unconvinced. Interestingly, he espouses a revisionist agenda, and does not develop his objections based upon Natural Law. There is almost no discussion of the Catholic position, based on Sacred Sripture and Thomistic theology.
One feels that Dr John, in making his claim for stable same-sex relationships, is seeking to make a pragmatic theology of mariage fit his desire for recognition of 'civil partnerships'.
Time will tell if he will convince those who are uncomfortable with his thoughtful but provocative ideas. If he becomes the Bishop of Bangor, then he will have a bigger platform to promote his agenda.