# 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"