Genesis 2:24
22 Then the Lord God made a woman from the part he had taken out of the man, and he brought her to the man. 23 Then the man said, “This one at last is bone of my bones and flesh of my flesh; this one will be called ‘woman,’ for she was taken out of man.” 24 That is why a man leaves his father and mother and unites with his wife, and they become one family. 25 The man and his wife were both naked, but they were not ashamed.
Matthew 19:5
3 Then some Pharisees came to him in order to test him. They asked, “Is it lawful to divorce a wife for any cause?” 4 He answered, “Have you not read that from the beginning the Creator made them male and female, 5 and said, ‘For this reason a man will leave his father and mother and will be united with his wife, and the two will become one flesh’? 6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.” 7 They said to him, “Why then did Moses command us to give a certificate of dismissal and to divorce her?”
Notes and References
"... Genesis 2:24 is responding to the question of why a person would leave the closest biological relationship (parents to children) in order to forge a relationship with a biological outsider. The answer offered is that marriage goes beyond biology to recover an original state, for humanity is ontologically gendered. Ontology trumps biology. This has shown Adam that the woman is not just a reproductive mating partner. Her identity is that she is his ally, his other half. We can now see that Genesis 2:24 makes more of a statement than we had envisioned. Becoming one flesh is not just a reference to the sexual act. The sexual act may be the one that rejoins them, but it is the rejoining that is the focus. When Man and Woman become one flesh, they are returning to their original state. (This also makes much better sense of Matthew 19:5-6 / Mark 10:7-8, 1 Corinthians 6:16-17 and especially Ephesians 5:31, where Paul is talking about being members of one body. Ontology is more central to this discussion than sex is. Genesis 2:24 may therefore have less to say about the institution of marriage and the nature of marriage than has been commonly thought) ..."
Walton, John H. The Lost World of Adam and Eve: Genesis 2-3 and the Human Origins Debate (p. 65) IVP Academic, 2015