Premarital agreements are just that- a contractual agreement between two people about to marry prior to their marriage. The laws affecting such agreements underwent a drastic rewrite after the Supreme Court of California's decision in a case entitled In Re Bonds regarding a premarital agreement Barry Bonds and his wife Susann had entered into the day prior to their marriage when he was earning $110,000 with the Pittsburgh Pirates. The legislature responded with constraints and requirements detailed requiring, for example, that the person waiving spousal support be sufficiently proficient in the language in which the provisions were explained. That might well have changed the result in the case had it been the law when Susann, born in Sweden, contested the agreement.
There is no one who should consider or enter into a premarital agreement without an attorney.
When parties marry in California they have a fiduciary duty to each other of the highest level and that maintains even through a divorce until the judgment itself. So postmarital agreements are of a different nature, requiring that fiduciary duty be honored in the financial disclosures and terms of the agreement.
Prenuptial and postmarital agreements should have attorney's representing each party- I doubt even Abraham Lincoln would have taken on either solo.