Marriage fulfils, and so makes sense of, a feature of our common human biological nature. Human beings come into existence with a dimorphically differentiated sexuality, clearly ordered at the biological level towards heterosexual union as the human mode of procreation.
It is not possible to negotiate this fact about our common humanity; it can only be either welcomed or resented. Marriage, precisely by being organized around this fact, enables us to welcome it and to acknowledge it as a part of God’s creational gift. It therefore enables us to be Christians, who believe in the goodness of creation, rather than Manichaeans who do not. We learn through marriage to rejoice in the fact that humankind is sexually dimorphic and heterosexually procreative, because within marriage this non-negotiable biological datum enables us to form relationships of love, between husband and wife, parent and child. What marriage can do, which other relationships cannot do, is to disclose the goodness of biological nature by elevating it to its teleological fulfilment in personal relationship. Other relationships, however important in themselves and however rich in intimacy and fidelity, do not disclose the meaning of biological nature in this way. They float, as it were, like oil upon water, suspended upon bodily existence rather than growing out of it.
It is clear why Christian understanding of marriage cannot be expressed solely in terms of relationship between persons. It is not that we can do without speaking of relationships and persons, but that this is only one of the two poles around which a Christian theology of marriage must move. To abstract this pole from the other is to deprive Christian thought of a movement which is essential to it, the demonstration that that which is distinctively human, the ‘personal,’ belongs most securely within the context of creation as a whole…. But a conception of marriage that abstracts the personal from the biological leaves the meaning of the biological order ambiguous, even questionable. Whereupon the temptation soon overtakes us to regard it as an arbitrary and pointless limitation on personal freedom which is better resisted.
Oliver O’Donovan, Transsexualism: Issues and Arguments (Cambridge: Grove Books, 2007), 6-7