Jesus is tested by the Pharisees on marriage and divorce. Thus he will not give an exhaustive teaching on the subject. He’s simply answering his inquisitors. Another will come later and “add” to what Jesus teaches here.
They ask if a man can divorce his wife for any reason (3). In many ancient societies, this would have been the common understanding of things. Jesus answer is revolutionary and instructive to us on many levels.
First, he goes back to the fundamental truths about marriage, given not in the law of Moses, but in the book of Genesis (4-6). To Jesus, what Genesis establishes is binding for all time.
Secondly, the fundamental truths about marriage have to do first with created realities. Marriage was a product of God’s designing His image-bearers in the form of two genders: male and female (4). Thus marriage is between a man and woman, because they were created to fit together in this bond.
Third, they fit together into such a bond that they become “one flesh” (5). This is all God’s doing, from the very beginning of creation. It’s all by God’s design. Therefore, what right does man have to separate from his Creator put together (6)? None.
But Moses allowed seemingly unlimited divorce, they claim (7). True, Jesus said. But only in order to deal with the hardness of heart of the people. Otherwise even greater evils would have occurred (8). So Jesus commands, on the basis of Genesis: no divorce, except for the sin of adultery (9).
Why does Jesus give this exception? Again, because of the definition of marriage, given by its Designer. When the one-flesh union is broken by adultery, the marriage covenant is so broken that that brokenness may be ratified by divorce, lest even greater evils occur. This is not required, but it is permissible.
The disciples, listening in, remark that it is better not to marry, if this is the only “out” (10). And Jesus does not talk them out of that conclusion.
Paul must have reflected on Matthew 19 when he added another exception in 1 Corinthians 7:15 - that of “abandonment.” Why did he feel free to “add” to Jesus’ teaching? It is because he too was considering Genesis and the other half of its definition - that of “cleaving” together in a singular relationship. Thus it is no coincidence that Paul too says that it’s better not to marry, if you can (1 Cor. 7:25-28).
God’s design for marriage is defined in Genesis. The glorious grace behind that design will appear later, in Christ and his Bride (Eph. 5:31-32). Thus it is binding for all, for all time.