While it is certainly true that the Gospel of Jesus Christ offers the world a resounding "YES!" to many things (forgiveness, redemption, salvation, life), it also confronts the world with an equally resounding "NO!" to those things contrary to God's purpose for creation. Archbishop Welby wishes to tone down the "NO!" and turn up the "YES!" because, well, it is just not fashionable for the church to be "against" so many things these days.
from John Wesley's final letter, to anti-slavery activist William Wilberforce, will provide a helpful historical illustration.
Unless the divine power has raised you up to be as “Athanasius contra mundum,” I see not how you can go through your glorious enterprise in opposing that execrable villainy, which is the scandal of religion, of England, and of human nature. Unless God has raised you up for this very thing, you will be worn out by the opposition of men and devils. But if God be for you, who can be against you? Are all of them stronger than God? O be not weary of well doing! Go on, in the name of God in the power of His might, till even American slavery (the vilest that ever saw the sun) shall vanish away before it.
It is never fashionable to be contra mundum, that is, against the world, but taking up the cross and following Christ is never about being fashionable. Wilberforce will never be remembered for anything except that he was against the wicked exploitation of other human beings through the slave trade. In his day, he was ridiculed, persecuted, and marginalized. But he persevered tirelessly and, in the end, prevailed because God had indeed raised him up to be as "Athanasius contra mundum."
It is a quite similar concern over the exploitation of other human beings, the scarring of the image of God in which they were created, that motivates so many "traditionalists" in the church today to oppose vigorously the legitimization of homosexual conduct and its domestication through the redefinition of marriage. It is at this very point that the world is right now opposing the Gospel. It is the responsibility of the Archbishop of Canterbury, of all people, to be "as Athanasius contra mundum" in the face of such opposition, shunning the ridicule and taking as sufficient encouragement the sure and certain knowledge that "if God be for you, who can be against you?".