The teaching on divorce appears in the Gospels of Matthew, Mark and Luke. Only Matthew’s Gospel contains the qualifier about unchastity. Mark and Luke do not.
But I say to you that every one who divorces his wife, except on the ground of unchastity, makes her an adulteress; and whoever marries a divorced woman commits adultery. (Matthew 5:32)
** He said to them, “For your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so**. And I say to you: whoever divorces his wife, except for unchastity, and marries another, commits adultery.” (Matthew 19:8-9)
And he said to them, “Whoever divorces his wife and marries another, commits adultery against her; and if she divorces her husband and marries another, she commits adultery.” (Mark 10:11-12)
"Every one who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery. (Luke 16:18
There was no divorce before Moses. Marriage was a covenant and it lasted until either the husband or wife died. This is the way it was “from the beginning”, which is an allusion to the way it was since Adam and Eve. God’s original plan was that marriage be forever.
Men would simply kill their wives or their wives would just disappear. Moses put a stop to this by allowing divorce temporarily. Divorce was a temporary fix for a bad situation, but it was never God’s plan.
Jesus returns marriage to the way it was “from the beginning”, a covenant that never ends.
-Tim-