..."priests who assume that people are too weak to honor their commitment to lifelong marriage.".....
Realistically, that is not merely an 'assumption', it is a fact and reality of many marriages. 
Sometimes such separation/divorce is even recommended by the Church in cases of spousal or child abuse. Sometimes it is just a 'fact of life' that some people are 'toxic' for each other.

'Ideals' are nice. Real life is sometimes something else.