Earlier quoted context omitted.
By definition, a prenuptial agreement only covers entering into the marriage. Future assets, especially in community property states, usually have to be specifically disclaimed by the other party in the marriage before they can be considered separate property. For example: A prenup can't specify that a house bought after marriage is separate property, but the non-owning spouse can voluntarily sign away rights to that…
>Why would a spouse be no less entitled to reap the rewards generated by the other spouse's start-up? Because just sitting around giving emotional support to someone doesn't entitle you to half of everything they get. It doesn't work that way with any other relationship (parent, friend, etc), so why should a marriage? This is especially true if both partners are working.
To answer your point more directly, one difference is that a marriage tends to produce children in a way that platonic friendships tend not do. My sense is that equal division of significant income and equity post-marriage will help, not hurt, children.
To answer your point a different way, the default marital property settings tend to work well for most people. Of course you can change those defaults to some extent with prenups, postnups, trusts, and so on. If you're still dissatisfied with your ability to alter those defaults, well, nobody's forcing you at gunpoint to marry someone you like to live with, have sex with, and even raise kids with.