en_tn_condensed/mat/19/05.md

He who made them also said, 'For this reason ... flesh.'

"And surely you know that God also said that for this reason ... flesh."

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

"they will become like one person"