It is operationally the same as being married, the relationships are not looser in any sense. Unless a longtime couple tells you about it, you often don't know. People assume you are married because you "look" and act how people expect married people to. Anecdotally, this is most common among people who were previously married, and therefore often have pragmatic and non-idealistic views of marriage, and both professionals; they already did it once and don't see the purpose or benefit. I know several couples that I discovered have been in this state for decades.

I do think this selects for relatively affluent couples who gain limited leverage by combining assets. For people with few assets, the benefits of combined economics are much higher.

Probably the biggest difference is that assets are not commingled by default and asset transactions do not require the signatures of both parties. Finances aren't just practically separate but also literally separate. Joint purchases are explicitly contracted as such. However, many well-off professional married couples also keep their finances approximately independent in practice, so this isn't unique though messier if they split up.

My situation is somewhat more interesting than most, but no I didn’t need to convince my partner(s) of any of this and everyone thinks of it the same way. That was one of the things that attracted me to the person with whom I have been living for the past several years.