In Matt. 19:3-12, we find a group of Pharisees attempting to trap Jesus into taking sides in a theological debate and stir up strife by asking Jesus to take a side in the Hillel-Shammai dispute that was raging at the time. The very popular and theologically liberal Rabbi Hillel taught that it was lawful for a Jewish man to divorce his wife for any cause whatsoever; the far less popular and theologically conservative Rabbi Shammai taught that it was lawful for a Jewish man to divorce his wife only if she had committed adultery against him. The Pharisees knew that Jesus was very conservative in His theology and they knew that He would most likely take the side of Rabbi Shammai, and consequently become, along with Rabbi Shammai, far less popular.
If the exception clause in Matt. 19:9 is genuine scripture rather than a very early addition to the original text, we have Jesus falling right into the trap set for Him by the Pharisees and taking the side of Rabbi Shammai. In my opinion, Jesus was not so foolish as to fall into such a trap, and that He replied to them,
9. And I say to you, whoever divorces his wife and marries another woman commits adultery.
That this was His actual reply is confirmed by the reaction of his disciples,
10. The disciples *said to Him, If the relationship of the man with his wife is like this, it is better not to marry.
This was not the reaction of the Jews to the teaching of Rabbi Shammai, so we can be quite certain that Jesus taught very differently, and did not include in His answer the exception for adultery that Rabbi Shammai included in his teaching. And, in His teaching elsewhere, Jesus is not quoted as including the exception for adultery having been committed.
Mark 10:11. And He *said to them, Whoever divorces his wife and marries another woman commits adultery against her;
12. and if she herself divorces her husband and marries another man, she is committing adultery.
Luke 16:18. Everyone who divorces his wife and marries another commits adultery, and he who marries one who is divorced from a husband commits adultery.
Compare:
Matt. 19:3. Some Pharisees came to Jesus, testing Him and asking, Is it lawful for a man to divorce his wife for any reason at all?
4. And He answered and said, Have you not read that He who created them from the beginning MADE THEM MALE AND FEMALE,
5. and said, FOR THIS REASON A MAN SHALL LEAVE HIS FATHER AND MOTHER AND BE JOINED TO HIS WIFE, AND THE TWO SHALL BECOME ONE FLESH?
6. So they are no longer two, but one flesh. What therefore God has joined together, let no man separate.
Paul, in his First Epistle to the Corinthians, expressed the same teaching in this manner:
1 Cor. 7:10. But to the married I give instructions, not I, but the Lord, that the wife should not leave her husband
11. (but if she does leave, she must remain unmarried, or else be reconciled to her husband), and that the husband should not divorce his wife.
We also find Paul applying this Biblical truth in Romans 7:
1. Or do you not know, brethren (for I am speaking to those who know the law), that the law has jurisdiction over a person as long as he lives?
2. For the married woman is bound by law to her husband while he is living; but if her husband dies, she is released from the law concerning the husband.
3. So then, if while her husband is living she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress though she is joined to another man.
4. Therefore, my brethren, you also were made to die to the Law through the body of Christ, so that you might be joined to another, to Him who was raised from the dead, in order that we might bear fruit for God.
(All quotations from the Scriptures are from the NASB, 1995)