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General Information:
In verse 5, Jesus quotes from Genesis to show that a husband and wife should not divorce.
He who made them also said, 'For this reason ... flesh.'
This is part of what Jesus expected the Pharisees to have understood from the scripture. The direct quotation can be expressed as an indirect quotation. Alternate translation: "And surely you know that God also said that for this reason ... flesh" (See: rc://en/ta/man/translate/figs-explicit and rc://en/ta/man/translate/figs-quotations)
For this reason
This is a part of the quotation from Genesis story about Adam and Eve. In that context the reason a man will leave his father and mother is because God created a woman to be the man's companion.
join to his wife
"stay close to his wife" or "live with his wife"
the two will become one flesh
This is a metaphor that emphasizes the unity of a husband and a wife. Alternate translation: "they will become like one person" (See: rc://en/ta/man/translate/figs-metaphor)