This is why gays have fought for the right. They got tired of co-mingling their finances, buying a house together and then having one partner thrown out into the street with little more than the clothes on their back when one of them died and the parents (or nearest blood relative) inherited the house.
This is a lot less obvious in practice than it seems. Fair-weather 'partnership' isn't really rare. Unless it has been tested at times of adversity it is hard to predict how people would react. A piece of paper does not ensure that the relationship will be anything but of the fair-weather variety, with one ready to jump ship at first signs of bad times, it does give one an opportunity to reflect on the gravity of the promise and vows and the public nature of the promise gives it some air of barrier to dissolution.
Sounds to me you're describing the whole point of marriage as a contract, as it specifies the default set of conditions and requirements that are applied to both parties, and also the rights and obligations of both parties in case of a contract resolution and even breach of contract.
> This is why prenuptial agreements can sometimes be a touchy subject, since it breaks that assumption of complete trust.
Again, it sounds like you're describing the whole point of marriage as a contract, as its the original (and baseline) prenuptial agreement.
Just ask Jeff Besos!
"Marriage is a contract" isn't just a catchphrase, it's literally a contract and serves exactly the same purpose as a business contract.
See how this statement sounds: "If 2 businesses are going to keep doing business together, a piece of paper telling them officially doing business together doesn't make any difference and if 2 business are going to break their engagements, some paper's likely not going to stop it"
In a sense everything above is true, but yet, a businesses wouldn't enter an important business deal without a contract.
Marriage can be seen similarly, when you sign the contract you are doing so with expectations : "this person makes me happy, I'm ok combining forces to go further"/"this business partnership makes me increase my profits, I'm ok doing this deal with them"
Both marriages and business deal can go sour. People change, business constraints change. Getting the right provisions in a contract makes sense in both cases.
Both people getting married and businesses striking a deal don't enter the contract with the hope that they will have to break the contract.
Both know they can break the contract if needed, with effect somewhat known.