Marriage was instituted by God for the first couple. "For Adam no suitable helper was found. So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, He took one of the man's ribs, and closed up the place with flesh. Then the Lord God made a woman from the rib He had taken out of the man, and He brought her to the man. The man said, 'This is now bone of my bones and flesh of my flesh; she shall be called woman, for she was taken out of man.' For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh."  Genesis 2:21-24 (NIV)

Jesus quoted this scripture in agreement with God's proclamation. "Have you never read that He who created them male and female from the beginning, said, 'Hence a man shall leave his father and mother, and cleave to his wife, and the pair shall be one flesh"?  So they are no longer two, but one flesh.  What God has joined, then, man must not separate." Matthew 19:4-6 (Moffatt translation)