You are right that some prenups are not enforced, for example, if one spouse can show that there is not a full disclosure of the other spouse's assets in the prenup, she was not represented by an attorney in signing the prenup, or she was not given enough time to review the prenup before signing it.

Or, you could not get married.

Indeed! There are also people who choose to not get married because of tax reasons. I will write another article on the tax benefits and tax disadvantages on getting married. Stay tuned!

And commonlaw marriage states completely ruin that plan in some cases.

There are like three states that have common-law marriage and it's not that easy to end up in one.

https://www.hg.org/article.asp?id=6782 -- here is the language around why a California prenup wouldn't be enforceable