What does Matthew 19:6 mean?

Pharisees have questioned Jesus about when it is lawful for a man to divorce his wife. The implied question is whether she needs to be guilty of infidelity to him or if he can simply decide that he does not like her anymore (Matthew 19:1–5).

Jesus, as He almost always does, replies to the Pharisees' question by pointing them to the Scriptures they know so well. He has declared, from Genesis, that marriage was God's design for humanity from the very beginning of creation. He made us male and female, after all (Genesis 1:27). God, before sin even entered the world, described the natural state of things as a man leaving his parents and holding fast to his wife, becoming one flesh with her (Genesis 2:24). Mysteriously, and as partly demonstrated through sex, marriage connects two people so closely together that they become one flesh, one person, in the eyes of God.

Now Jesus drives the point home. Marriage is not something humans have made for themselves by tradition and custom. God does this. He joins a man and woman together in this way. And what God has joined no mere human should dare to divide or separate. Jesus will explain an important exception to this in the following verses (Matthew 19:9), but He begins with the deepest and truest intent of the heart of the God. Those God joins in marriage are not meant to be separated by divorce.