It’s become routine to say that current Anglican disputes are really about scriptural authority or unity of doctrine within the Anglican communion rather than sex. To my mind that doesn’t wash. At bottom the basic issue is always truth, in this case the truth about sex. If the right answer is that its human meaning has no intrinsic connection to natural function, so that it becomes what we make of it, then that will also be the correct interpretation of scripture and ultimately the only acceptable basis for unity of doctrine.
It seems rather that the issue is what might be called Christian physics—the nature of the physical world—and in particular Christian anthropology—the nature of man as embodied. It is basic Christian doctrine that the physical world expresses higher things. God made the world and called it good, and the heavens therefore proclaim his glory. Even more pointedly, the Incarnation demonstrates that the human body can fully express the divine. People could see and hear Christ and recognize him as God.
Sex is basic to human life and extraordinarily expressive. To dissociate its significance from its physical and vital function is to open a gap between grace and the natural world that seems unbridgeable. If the extraordinarily expressive sexual aspects of the human body express nothing very definite, even in relation to sex, so that what you do with them is entirely up to you, then how can the human body possibly be expressive enough for the Incarnation to be real and recognizable? It may be possible to believe such a thing, but to my mind it doesn’t sit very well.