Genesis 2:18-24 NIV84

It is not good for man to be alone. God created and presented the animals to Adam, but a suitable helper couldn't be found in them (20). God made a woman from the rib of Adam to be a partner with Adam.

Adam says "This is now bone of my bones and flesh of my flesh..."
So therefore, that is why a man will leave his father and mother and be united to his wife, and they will become ONE in flesh (again).