The problem with a pre-nup agreement is that there is a logical assumption that the couple has reason to believe the marriage won't last, and that separation or divorce is a possibility. Therein lies the conflict about the marriage covenant (as opposed to a contract). The marriage vows are intended to be permanent in the eyes of the Church (founded upon God's law and teachings).
Everything Ray writes is what I have heard about prenups. I am an estate planning attorney. A trust for husband and wife, each, can provide for their lives and the remainder to children from a previous marriage. Problem solved. I am a convert, but my understanding is that a prenup is a condition on the marriage. This condition is the fast tract to annulment, I have heard. It is also my understanding that a priest should not marry a couple if there is a pre nup. Personally as a Catholic attorney, I feel that I am not able to prepare them for couples. I always refer them to someone else.