What God has joined

In Jesus' day Jewish teachers debated when a man was allowed to send his wife away. Some held that it was already permitted if she had burnt the food. A woman had little protection.

Jesus doesn't join that debate. He goes back to the beginning, to creation: the two become one. It is a high ideal. It is also a protection of the weaker party. And for anyone who has seen a marriage fail, there is always grace with Jesus too.