First, by the words " except on the grounds of fornication."
[Found in Matthew's Gospel account ],

Jesus indicated that the marriage bond is not to be viewed lightly or broken easily.
The prevailing rabbinic teaching allowed for a divorce on such minor grounds as a wife's spoiling a dish of food or talking to a strange man.
Why, divorce was even allowed if a husband found a women who was more attractive to his eyes !
One Biblical scholar notes:

When Jesus spoke as he did he was strikeing a blow for women by
seeking to restore marriage to the position it ought to have.
" Indeed , marriage ought to be a permanent union in which a women can feel secure.
{ Mark 10:6-9 }