“But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.”
But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery. Jesus' teaching on divorce locates the moral weight not in the certificate but in the reality of the marriage — divorce that is not for sexual immorality does not dissolve the marriage in God's sight; it only creates the situation where both parties inevitably enter adulterous relationships. The sexual immorality exception (porneia) acknowledges that some situations are already broken in ways that make this teaching inapplicable. Genesis 2:24 says the two shall become one flesh — Jesus' divorce teaching is rooted in the creation design, which Moses' provision accommodated without establishing as the ideal.
Church Fathers on Matthew 5:32
The Lord had taught us above that our neighbour’s wife was not to be coveted, He now proceeds to teach that our own wife is not to be put away.
For touching Moses' allowance of divorce, the Lord and Saviour more fully explains in conclusion, that it was because of the hardness of the hearts of the husbands, not so much sanctioning discord, as checking bloodshed.
For when Moses brought the children of Israel out of Egypt, they were indeed Hebrews in race, but Egyptians in manners. And it was caused by the Gentile manners that the husband hated the wife; and if he was not permitted to put her away, he was ready either to kill her or ill-treat her. Moses therefore suffered a bill of divorcement, not because it was a good practice in itself, but was the prevention of a worse evil.