There are some things you can’t or simply should not include in a premarital agreement. Some will simply be unenforceable, but depending upon other terms of the agreement, the inclusion may destroy the validity of any of the terms of the agreement! The terms that should be avoided fall into the categories of illegal; void as against the public policy of the state; and those that are determined to be promotive of divorce.
Agreements to act in any unlawful enterprise have never been enforceable in a court of law, so it is no different if you slip one in your premarital agreement. It will not be enforced.
Those agreements that are void as against public policy include any attempt by way of the agreement to define what would happen in the event of children born to or adopted during marriage by both parties. No state will honor any predetermined scheme as the court acts as a guardian of the rights of the children and it will determine disputes regarding parentage, custody, visitation and support based on the facts that exist at the time of the dispute and the law of the state.
Some states, California is not included, prohibit the waiver of a claim of spousal support or alimony. California allows for support waivers but adds a few requirements that pretty much guarantee that each party has to be separately represented by an attorney for it to be enforceable later.
Provisions that promote divorce include just about any agreement to impose a financial or other penalty on a party for conduct. This has applied to provisions that a financial penalty if there is a divorce for such things as marital infidelity; financial penalty for drug use. It has also applied to financial rewards for a divorce such as a dowry or ketubah. Additionally, provisions that purport to impose any obligation with respect to religious practices particularly for the children.
Because of the potential for pitfalls in selecting provisions for an agreement, it is often best advised that you obtain professional guidance if you desire the agreement to be of any value.