19:4 Have ye not read. In response to a vital question about the most important of all human institutions (marriage and family), as His authority the Lord quotes the account of creation in Genesis. It is obvious that He—contrary to the opinion of modern intellectuals and liberal theologians—regarded the creation record as historically true and divinely inspired.
19:5 cleave to his wife. Modern critics often allege that the first two chapters of Genesis are two different and contradictory accounts of creation. Jesus, however, quoted from Genesis 1:27 and Genesis 2:24 as perfectly complementary and of absolute authority. Furthermore, He was there at the beginning!
19:6 let not man put asunder. The creation account is thus the foundation for the institution of marriage, which was validated by the Creator Himself and established to be a lifelong union between one man and woman, who were commanded to “be fruitful and multiply” (Genesis 1:28).
19:7 writing of divorcement. Deuteronomy 24:1-4. Christ here confirms the Mosaic authorship of Deuteronomy.
19:9 except it be for fornication. The relatively easy divorce allowed in the Mosaic law was not intended in God’s original economy (Matthew 19:8). The only allowable grounds for divorce according to Christ is adultery. The Greek word for “fornication” (porneia) could include any sexual sin committed after the betrothal contract. On the other hand, Jesus did not even allow this exception in Luke 16:18. The conclusion would be that, even though adultery or fornication are permissible grounds for divorce and remarriage, it is still better to salvage the marriage if possible. See also Romans 7:2-3; I Corinthians 7:10-15.