Marriage was meant to be 'permanent' in Jesus' time because He knew:  (1)  The vast majority of people, at that time, didn't live very long.  (They knew nothing about sanitation and nutrition.)  (2) Men could divorce their wives but the women couldn't divorce their husbands.  And, whether or not that is a fact (it is somewhat debatable), an absolute fact was and is that divorced women were condemned to a live of abject poverty if they didn't remarry.  Jesus was protecting women.