As a canon lawyer, I have reviewed pre nups, and would verify 1ke’s response in general. They need careful analysis and can sometimes be acceptable.
At other times, they do reflect the dangers that Brother Rich presents. The you- know- what is in the details and the manifested intention of the parties, and I would be hesitant to make a blanket assertion about all of them.
However, conditional consent contingent on a future event is not the only possibility which they pose. They may also pose the possibility of an exclusion of an essential property or element of marriage noted in canon 1101. It is also possible that even another kind of deficit of consent may present an underlying motive for them.
So they need careful canonical and even civil examination by qualified experts.
Certainly, they present an exceptional turn of events, since marriage is to be a consortium (English: partnership) of the whole of life. The Latin origin of consortium is the “casting of lots” together. While marriage is not a toss of the dice but an institution established by God to which people are joined by consent, it is a complete sharing of life that involves an unknown future.