For this reason, a man will leave his father and mother and cleave to his wife, and the two shall become one flesh.
– Genesis 2:24 (KJV)
“The Genesis text calls what happens ‘cleaving.’ This archaic English term, which you can find in the King James Version, conveys the strength of the Hebrew word which modern translations render: ‘united to.’ It is a Hebrew word that literally means to be glued to something. Elsewhere in the Bible, the word ‘cleave’ means to unite to someone through a covenant: a binding promise or oath. Why do we say that marriage is the most deeply covenantal relationship? It is because marriage has both strong horizontal and vertical aspects to it. … [when exchanging vows during a wedding ceremony, notice that the bride and groom are speaking to the minister, not to each other.] What they are really doing is making vows to God before they turn and make vows to each other. They are speaking vertically before they speak horizontally.”
– Chapter 3: “The Essence of Marriage”
“Marriage is a union between two people so profound that they virtually become a new single person. The word ‘united,’ in older translations, to cleave, means to make a binding covenant or contract. This covenant brings every aspect of two persons’ lives together. They essentially merge into a single legal, social economic unit. They lose much of their independence. In love they donate themselves wholly to the other. To call the marriage ‘one flesh,’ then, means that sex is understood as both a sign of that personal, legal union and a means to accomplish it. The Bible says ‘don’t unite with someone physically unless you’re also willing to unite with the person emotionally, personally, socially, economically and legally. Don’t become physically naked and vulnerable to the other person without becoming vulnerable in every other way because you have given up your freedom and bound yourself in marriage.’ Then once you have given yourself in marriage, sex is a way of maintaining and deepening that union as the years go by. … Indeed, sex is perhaps the most powerful God-created way to help you give your entire self to another human being. Sex is God’s appointed way for two people to reciprocally say to one another ‘I belong completely, permanently and exclusively to you.’ You must not use sex to say anything less.”
– Chapter 8: “Sex and Marriage”
From Tim Keller’s The Meaning of Marriage: Facing the Complexities of Commitment with the Wisdom of God