Commentaries:
Notice what Jesus did in this instance: He leap-frogged over Deuteronomy 24, which covers divorce and remarriage and sets a foundation of principles from which divorce-and-remarriage decisions can be made. Jesus skipped right over this passage in the midst of Israel's civil law. In fact, He skipped over the whole Old Covenant, reaching all the way back to Genesis 2 for His authority for a judgment regarding marriage and divorce.
There is instruction here. Even though God permitted them to divorce and remarry because of their unconversion (hardness of heart), the higher and greater authority—the standard—where God originally established His intention. This is a clear example from Christ: The higher and greater authority lies in God's originally established intention.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Nineteen)
Other Forerunner Commentary entries containing Matthew 19:8:
Genesis 2:21-24
Matthew 12:9-14
Matthew 12:9-14
Mark 3:1-5