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. AT: "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]])
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.
This is a metaphor that emphasizes the unity of a husband and a wife. AT: "they will become like one person" (See: [[rc://en/ta/man/translate/figs-metaphor]])