With regards to all the finance and security side of the argument, you are suggesting that people get married to mend/avoid the problems that the institution of marriage has created.
Marriage created the idea of a woman being financially dependent on a man. First it created the idea of wife as property, the rule of thumb, people as chattels. Then. it created the idea of joint bank accounts, mortgages, merging finances, having the woman give up work, and because of marriage the laws surrounding certain benefits, inheritances etc were set up.
Marriage is what made women vulnerable in the first place, trapping them in to a marriage, denying them control over the finances, without the laws to protect them.
Without the conventions of marriage historically, there would not be the automatic assumption of merging finances and spousal interdependence. Each financial step would be taken individually with advice, just as it would be in a business partnership, for example, in which there is no such social convention and history. And that would be mutually beneficial in nearly every case. At the very least, each person would be informed as to the risks and benefits of each financial decision transferring money or power to another person.
Citing marriage as a sensible step to avoid 'having to do all that' is ridiculous. SAHP and dependent spouses are in that position because they have surrendered financial viability and independence in return for a promise that they will be taken care of - marriage vows - and the merging of lives and assets that takes place upon marriage. Yet marriage vows can be broken in the blink of an eye without signing any legal paper.
Unmarried partners have to take individual steps in order to merge their lives and assets, steps upon which they generally have to seek advice, legal or otherwise, giving them more information about the risks, not less.
Vulnerable people out there are vulnerable regardless of their marital status.
I am a SAHP. But I am not financially dependent on my partner. I have a seperate bank account, a seperate savings account, a joint mortgage, joint parental responsibility, joint wills, joint bills, guardianship, LPAs, pensions, and life insurances. We are not rich enough for CGT and IHT and would struggle to acheive those thresholds individually in the next 50 years.
I would be far more vulnerable married and figuring that would sort it all out.