Half the men in Seattle are never-married singles, census data shows

Anecdotally, among the people I know in Seattle, many people who have happily been in the same relationship for decades are not married. People are not avoiding long-term relationships, they are avoiding the baggage and fairly rigid assumptions that comes with state intervention in their relationships. There is zero social pressure to be “officially” married so people have no reason to do it for the sake of social conformity. Both men and women are subscribing to this.

I think some of this is a side-effect of many people planning to never have children.

What is US law on this and what are the "rigid assumptions" people find troublesome.

In the UK the main effect of marriage is that it protects the lower earning partner if the relationship breaks down - most often (even now!) this is a woman who gives up work/takes a break from work to bring up kids. I come across a good many single mothers (mostly online because of an FB group I admin) who would have been MUCH better off financially if they had been married, and a lot who are a lot better off because they were married.

Historically the main reason for legal marriage was to protect women from being left with kids by feckless men who evaded responsibility. Its a bit less pressing now we have paternity tests (and contraceptives) but its still a problem, and whoever gives up career for childcare still loses out without marriage.

There is minimal US law on this. States have their own laws individually which vary considerably. The context here has nothing to do with children and these people commonly don’t have any. Legal jurisdiction can be quite fluid, which just adds to the complexity.

A major motivating factor is that marriage can create myriad ruinous financial entanglements and eliminate most possibilities for recourse. Especially in affluent places like Seattle, people are often entering these relationships with a mature financial and business life. The loss of independence under law, both technically and in practice, in regard to these affairs in marriage has produced countless examples of bad outcomes for people, often forcing them to start over from zero in their 30s or 40s. It doesn’t matter if you divorce later, the damage is already done. By not getting married, they retain independence in these matters recognized by law which shields them from the downside scenarios.

These cases are mostly about retaining the ability to continue to run their businesses, investments, etc as they wish both in marriage and after. States in the western US are notoriously much less accommodating of protecting the interests of individuals in marriage than in some other parts of the country (purportedly due to the strong influence of Spanish legal customs in that region of North America). The forced loss of agency is not compatible with many requirements of competently living a modern life.

I probably know more women who were ruined by this in marriage than men. It has nothing to do with staying home or quitting a career to raise kids.

I do not really understand that. Why would people have to start again from zero? What sort of "forced loss of agency" happens? Are there laws allowing people to stop their spouse from running an existing business?

It comes from an ancient legal regime called “community property” inherited from the Spanish in the southwestern US. The details of what is within legal scope and how it is handled varies significantly with jurisdiction. In most cases the loss of ownership independence also comes with a loss of management independence — your spouse can get involved and has veto power. Assets that are out of scope can bleed into scope over time. The entire notion interacts poorly with modern legal and financial structures related to businesses and assets.

It isn’t talked about much but I’ve seen a couple startups destroyed by this and it is a not uncommon source of dead equity in cap tables.