Then why are there pre-numps?
There shouldn't be.
Not if the contract forbids it, and marriage by definition is a life-time commitment.
Find me one U.S. or Canadian civil marriage contract that forbids divorce.
Oh, and about that definition:
mar·riage
Pronunciation: 'mar-ij
Function: noun
1 : the state of being united to a person of the opposite sex as husband or wife in a legal, consensual, and contractual relationship recognized and sanctioned by and
dissolvable only by law —see also DIVORCE
2 : the ceremony containing certain legal formalities by which a marriage relationship is created
marriage
n 1: the state of being a married couple voluntarily joined for life (or until divorce)*
*did not add