It's clearly a stretch for our current culture to understand why Christian thinkers have (until recently) held that certain pleasurable acts between consenting adults are nevertheless wrong. Western society has grown intensely individualistic in theory even as technology has rendered us intensely communitarian in practice. A contemporary secular understanding of sexuality will therefore be big on individualism, and any individualistic account of sexuality in the age of birth control will necessarily be weak on "sin". Sex outside of marriage no longer automatically implies pregnancy, danger, expense, shame and poverty. (I should note, though, that even when birth control is involved the risk of unplanned pregnancy is hardly negligible: it's about the same as your chances of dying when climbing Mt. Everest. The impact isn't quite the same, but as anyone can tell you who has ever raised a baby, or had an abortion, or placed a child for adoption, it's substantial.)
Perhaps the fact that sex is less objectively risky these days should change how we view it; but perhaps not. Even under a purely secular thesis of evolutionary psychology, we're wired to think of sex as producing children, i.e., whether we realize it or not, having sex with someone reserves a place for them in our brain that says, "This person is important: I may be seeing their genes in my children each day for the next 18 years."
But of course, in Rowan Williams' essay "The Body's Grace", he's trying to speak, not as an unencumbered, post-modern, secular self, but as a Christian, indeed, an Archbishop, and from within a long Christian tradition of reflection on marriage and sexuality. It's therefore odd that Williams' account of marriage is entirely individualistic, and in fact seems to owe more to Locke's idea of a "social contract" (with a few psychological glosses) than to any Biblical framework. Williams does think that long-term commitment is important, but his explanation as to why this is so ends where one partner's body connects to the other. At least in this essay, for Williams the only parties concerned in a given marriage are the two individuals. This is the only explanation he gives for why two partners should remain faithful to each other:
I can only fully discover the body's grace in taking time, the time needed for a mutual recognition that my partner and I are not simply passive instruments to each other. Such things are learned in the fabric of a whole relation of converse and cooperation; yet of course the more time taken the longer a kind of risk endures. There is more to expose, and a sustaining of the will to let oneself be formed by the perceptions of another. Properly understood, sexual faithfulness is not an avoidance of risk, but the creation of a context in which grace can abound because there is a commitment not to run away from the perception of another.
It's somewhat strange that Williams neglects the communal dimension of marriage. A stable family (and therefore marriage, and therefore sex) is a community affair, and marriage is thus an institution in which society has a quite legitimate and vested interest. But more to my point, I think it's especially odd that his essay neglects "covenant faithfulness", a Biblical theme which features prominently in Christian accounts of marriage throughout history.
God's people have consistently understood their relationship to God through the lens of "covenant". This tradition goes back to the story of Noah, reaches through Abraham, Sinai, and finally to the "new covenant in my blood". This "new covenant" was predicted by Jeremiah, instituted at the Last Supper and on the cross, and is formally re-enacted each Sunday through the Eucharist in churches around the world.
In addition, this covenant between God and his people is understood through the lens of marriage, the covenant between a man and his wife. There are many passages where this image comes to the fore, but nowhere is it more profoundly expressed than in the book of Hosea:
When the LORD began to speak through Hosea, the LORD said to him, "Go, take to yourself an adulterous wife and children of unfaithfulness, because the land is guilty of the vilest adultery in departing from the LORD." (Hosea 1:2)
In the New Testament, the relationship is reversed: not only is light shed on the divine covenant through the human covenant of marriage, but new light is shed on the nature of marriage by observing God's covenant with us.
Wives, submit to your husbands as to the Lord. For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior. Now as the church submits to Christ, so also wives should submit to their husbands in everything.
Husbands, love your wives, just as Christ loved the church and gave himself up for her to make her holy, cleansing her by the washing with water through the word, and to present her to himself as a radiant church, without stain or wrinkle or any other blemish, but holy and blameless. In this same way, husbands ought to love their wives as their own bodies. He who loves his wife loves himself. After all, no one ever hated his own body, but he feeds and cares for it, just as Christ does the church— for we are members of his body. "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh." This is a profound mystery—but I am talking about Christ and the church. However, each one of you also must love his wife as he loves himself, and the wife must respect her husband. (Ephesians 5:22-33)
Covenant seems to be an example of the analogia entis, of the enlightening similarities which exist between God and His creation. We begin to understand the faithfulness which exists in God's own nature as we observe His faithfulness to His people through history, but also as we experience the faithfulness of one spouse to another in marriage.
Christian thinkers have universally understood marriage in this way, as a human covenant of divine origin, which is thus a suitable image of the covenantal relationship between God and His people. They have differed as to whether it is a sacrament or merely a divine institution, but none have sought to disagree with Paul's assessment that it is a mystery and a symbol of Christ's love for the Church.
If the husband die, with whom a true marriage was made, a true marriage is now possible by a connection which would before have been adultery. Thus between the conjugal pair, as long as they live, the nuptial bond has a permanent obligation, and can be cancelled neither by separation nor by union with another. But this permanence avails, in such cases, only for injury from the sin, not for a bond of the covenant. In like manner the soul of an apostate, which renounces as it were its marriage union with Christ, does not, even though it has cast its faith away, lose the sacrament of its faith, which it received in the laver of regeneration. (Augustine, On Marriage and Concupiscence)
Matrimony is instituted both as an office of nature and as a sacrament of the Church. As an office of nature it is directed by two things, like every other virtuous act. One of these is required on the part of the agent and is the intention of the due end, and thus the "offspring" is accounted a good of matrimony; the other is required on the part of the act, which is good generically through being about a due matter; and thus we have "faith," whereby a man has intercourse with his wife and with no other woman. Besides this it has a certain goodness as a sacrament, and this is signified by the very word "sacrament." (Thomas Aquinas, Summa Theologica)
The third incomparable grace of faith is this, that it unites the soul to Christ, as the wife to the husband; by which mystery, as the Apostle teaches, Christ and the soul are made one flesh. Now if they are one flesh, and if a true marriage—nay, by far the most perfect of all marriages—is accomplished between them (for human marriages are but feeble types of this one great marriage), then it follows that all they have becomes theirs in common, as well good things as evil things; so that whatsoever Christ possesses, that the believing soul may take to itself and boast of as its own, and whatever belongs to the soul, that Christ claims as his. (Luther, First Principles of the Reformation)
But in order to press the matter more on the priests, he calls their attention to the fact that God is the founder of marriage. Testified has Jehovah, he says, between thee and thy wife. He intimates in these words, that when a marriage takes place between a man and a woman, God presides and requires a mutual pledge from both. Hence Solomon, in Proverbs 2:17, calls marriage the covenant of God, for it is superior to all human contracts. (Calvin, Commentary on Malachi)
It is the divine intention that persons entering the marriage covenant become inseparably united, thus allowing for no dissolution save that caused by the death of either husband or wife. (Westminster Confession of Faith)
In this covenant of grace, we may see the cream of God's love, and the working of his bowels to sinners. This is a marriage covenant. "I am married to you, saith the Lord." (Thomas Watson, Body of Divinity)
In addition to marriage, the Bible also provides various resources for thinking more specifically about sex. Its approach is surprisingly multifaceted, much more so than many modern thinkers. Sex is for procreation (Genesis 2:28), for pleasure (1 Corinthians 7:5, the entire Song of Solomon), and for comfort (Genesis 24:67; 2 Samuel 12:24). But primarily, sex is explained as the sign and seal of the marriage covenant, and in this way, sexual fidelity is a symbol or image of God's covenantal faithfulness to us.
Then I heard what sounded like a great multitude, like the roar of rushing waters and like loud peals of thunder, shouting:
For our Lord God Almighty reigns.
Let us rejoice and be glad
and give him glory!
For the wedding of the Lamb has come,
and his bride has made herself ready.
Fine linen, bright and clean,
was given her to wear." (Fine linen stands for the righteous acts of the saints.)
No longer will they call you Deserted,
or name your land Desolate.
But you will be called Hephzibah,
and your land Beulah;
for the LORD will take delight in you,
and your land will be married.
As a young man marries a maiden,
so will your sons marry you;
as a bridegroom rejoices over his bride,
so will your God rejoice over you.
For this reason, sex outside of marriage, especially adultery, is identified directly as faithlessness to God.
During the reign of King Josiah, the LORD said to me, "Have you seen what faithless Israel has done? She has gone up on every high hill and under every spreading tree and has committed adultery there. I thought that after she had done all this she would return to me but she did not, and her unfaithful sister Judah saw it. (Jeremiah 3:6)
The adultery of which Jeremiah speaks is both literal and figurative: the inhabitants of Israel and Judah were having sex with sacred prostitutes and thus were unfaithful to their spouses, and because they were doing this in disobedience to God's command, and were worshiping other gods, the same act showed their unfaithfulness to God. The same identification of faithlessness to divine and human spouses is made in Ezekiel:
The LORD said to me: "Son of man, will you judge Oholah and Oholibah? Then confront them with their detestable practices, for they have committed adultery and blood is on their hands. They committed adultery with their idols; they even sacrificed their children, whom they bore to me, as food for them. They have also done this to me: At that same time they defiled my sanctuary and desecrated my Sabbaths. On the very day they sacrificed their children to their idols, they entered my sanctuary and desecrated it. That is what they did in my house. (Ezekiel 23:36-39)
Or as Paul puts it in 1 Corinthians 6:15-20:
Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, "The two will become one flesh." But he who unites himself with the Lord is one with him in spirit. Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body. Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your body.
The understanding which lies under all these passages is that sex is the sign and seal of the covenant of marriage, and that sex with anyone besides your spouse is a violation not only of your spouse's trust, but of God's. As the writer of Hebrews says:
Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral. (Hebrews 13:4)
Paul is very clear on this in 1 Thessalonians 4:3-8:
It is God's will that you should be sanctified: that you should avoid sexual immorality; that each of you should learn to control his own body in a way that is holy and honorable, not in passionate lust like the heathen, who do not know God; and that in this matter no one should wrong his brother or take advantage of him. The Lord will punish men for all such sins, as we have already told you and warned you. For God did not call us to be impure, but to live a holy life. Therefore, he who rejects this instruction does not reject man but God, who gives you his Holy Spirit.
The key is to note that sexual immorality is a wrong against one's brother, because it takes advantage of him (or her). Presumably part of the reason is because of the risk of unwanted pregnancy, but also because it's a violation of the marriage covenant (even if you're not yet married, it wrongs your future spouse); and in so violating the covenant with our partner, we violate our covenant with our God.
Anyone who thinks of marriage as a covenant can understand sex as the sign and seal of the marriage covenant, though of course, only Christians can understand it as a symbol or sacrament of the divine love of Christ for His Church. However, it seems that anyone who is a Christian should give pretty serious consideration to this particular understanding of sexuality. Certainly the New Testament teaches us that "sexual immorality" (typically πορνεια in Greek) is a serious sin, and the understanding of sexual fidelity as a symbol of divine fidelity helps to explain why its writers are so insistent on this point.
In other words, it's disappointing that Rowan Williams ignores this lengthy and venerable line of Christian thought on the relationship of sex to marriage and, indeed, to God. I wish I had a better explanation for this neglect than that if he had done otherwise, it would have shown the main point of his essay in rather a dubious light.