And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man. Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh (Genesis 2:23,24).
When the Pharisees asked for His view on divorce, Jesus replied by quoting our text, giving the Creators view on marriage and how men and women should approach it, if they are to function as they were designed (Matthew 19:4,5). He added, Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder (v.6).
In some special way, known fully only to Him, a man and woman in a marriage relationship can truly become one flesh, just as Adam and Eve were one flesh after Eve had been fashioned from Adams side. (Christs doctrine of marriage has no logical foundation, by the way, nor do we have any reason to marry, if Adam and Eve were not real, specially created people.)
Our text was also quoted by Paul as he more fully explained the marriage doctrine (Ephesians 5:31), prefacing it with a brief discussion of the relationship between the Lord and His Church (v.30). Just as we are inseparably members of His body, of His flesh, and of His bones, He designed each of us to be inseparably one flesh with his or her spouse.
Paul uses a forceful word for leave, meaning to completely leave ones parents and be joined to the spouse. This word is equally forceful and leaves no room for a half-hearted commitment.
Marriage partners, in the eyes of the Creator, should be inseparable, just as the bones and flesh of a body cannot be separated, and just as we cannot be separated from the love of God in Christ (Romans 8:3539). JDM