From the Desiring God website.
The most foundational thing we can say about marriage is that it is God's doing. John Piper explains, "A glimpse into the magnificence of marriage comes from seeing in God's word that God himself is the great doer. Marriage is his doing. It is from him and through him" (24).
In his book This Momentary Marriage, Pastor John gives four reasons why marriage is God's doing:
First, marriage was God's design.
While Genesis 1:27–28 makes clear that marriage is meant for male and female, the logic of Genesis 2 also confirms it.
In [Genesis 2:18], it is God himself who decrees that man's solitude is not good, and it is God himself who sets out to complete one of the central designs of creation, namely, man and woman in marriage. "It is not good that the man should be alone; I will make him a helper fit for him." Don't miss that central and all-important statement: God himself will make a being perfectly suited for him — a wife. (21)
Second, God gave away the first bride.
God took the role as the first Father to give away the bride. "Genesis 2:22: 'And the rib that the Lord God had taken from the man he made into a woman and brought her to the man.' He didn't hide her and make Adam seek. He made her; then he brought her." (22)
Third, God spoke the design of marriage into existence.
We can see this if we look carefully at Matthew 19:4–5: "[Jesus] answered, 'Have you not read that he [God] who created them from the beginning made them male and female, and said [Note: God said!], "Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh"'?" Jesus said that the words of Genesis 2:24 are God's words, even though they were written by Moses. (22)Fourth, God performs the one-flesh union.
The one-flesh union between a man and woman is the heart of what marriage is.
Genesis 2:24 is God's word of institution for marriage. But just as it was God who took the woman from the flesh of man (Genesis 2:21), it is God who in each marriage ordains and performs a uniting called one flesh. Man does not create this. God does. And it is not in man's power to destroy. This is implicit here in Genesis 2:24, but Jesus makes it explicit in Mark 10:8–9. He quotes Genesis 2:24, then adds a comment that explodes like thunder with the glory of marriage. "'The two shall become one flesh.' So they are no longer two but one flesh. What therefore God has joined together, let not man separate."
When a couple speaks their vows, it is not a man or a woman or a pastor or parent who is the main actor — the main doer. God is. God joins a husband and a wife into a one-flesh union. God does that. The world does not know this. Which is one of the reasons why marriage is treated so casually. And Christians often act like they don't know it, which is one of the reasons marriage in the church is not seen as the wonder it is. Marriage is God's doing because it is a one-flesh union that God himself performs. (23)