He who created them from the beginning MADE THEM MALE AND FEMALE, and said
FOR THIS REASON A MAN SHALL LEAVE HIS FATHER AND MOTHER AND BE JOINED TO HIS WIFE, AND THE TWO SHALL BECOME ONE FLESH. Matthew 19:4
So what is meant by "the two shall become one flesh?"
Here are my thoughts..
In the old testament the Tabernacle and later, the Temple, was the dwelling place of the essence of God on earth. It was the center of Spiritual life where believers could be closer to their maker. In the new testament our human bodies become The Temple where the Spirit "in dwells." A temple that, in its design puts those created by the human hands of the old testament to shame.
During the act of procreation, the male and female bodies are indeed conjoined as one, but this temporary union always ends with the death of the sperm and egg; unless a new beautiful human life is created. God's purpose for marriage is to bring life, not death.
The new birth, is a human being created by the "union" of the mother & father's unique DNA. "Therefore the two become one." This beautiful and unique union cannot be denied by any one. Not the father, the mother nor the child, no matter how much they may wish is to be so. It cannot be undone nor somehow separated back into it component parts.
Thereby, the reason, or the purpose, for the marriage, that is the child, cannot be undone. Through the legal fiction of divorce, we can pretend the reason no longer exists, but the flesh and bone, the reality of the child, still exists.
But in God's Kingdom, this new birth means much more than a new "human being." When a child is born, don't we believe that an "eternal being" is newly created at that moment? I can understand the science of how my wife and I can create a new human being...But I'm rather confident that I cannot create an eternal being. My wife is amazing. But I doubt even she can create an eternal being.
The birth is a life giving, eternal act of co-creation with our Maker, not a crude, temporary act of procreation between two worldly creatures.
And not only eternal, but an eternal being created in the image of God. And not just in God's image, but ultimately, God asks this "reason for marriage" to become his temple for the in-dwelling of The Holy Spirit. A sanctuary for God's presence on earth. The new place from which Spiritual life on this planet emanates. A place where God gets as close as possible to the heart of the believer.
Just as our ancestors, in the old testament, in faith carefully devoted themselves to build and prepare The Temple with the gifts given to them by God. Today, the mother and father are given the gift of a new life and are asked by God in faith, to carefully devote themselves to prepare this new living Temple for the in-dwelling of the Holy Spirit incarnate.
The Spirit, then becomes that child's Advocate, Counselor, Comforter and Intercessor for the glory of God on this earth and in His heavens. And once having received the sacrament of baptism, may at some point leave their mother and father, be wed to their spouse and be consummated by God to co-create a new eternal temple in God's image.
That is the high purpose and privilege of marriage.