In France we have civil partnerships (PACS)
It's gives cohabiting couples the broadly the same legal and tax status as married couples. Have to be cohabiting, cannot be related (!) and cannot be married or CP with anyone else, anywhere in the world. One of the couple has to be french.
We are protected by legal parts i.e. inheritance as tax here is super high without the CP. We pay more tax, although our situation is unusual, most couples benefit from shared tax benefits, for it's just one of the many types of french taxes has same thresholds for a couple as for one person. Our situation is unusual in younger couples though and often it's younger couples who like the PACS idea, it's practical, we are living together, if one partner passes away due to eating all the gateau, the other doesn't pay inheritance tax.
If I get bumped off by a frenchman, you all know why. The non french half of the couple doesn't qualify for citizenship through marriage with a CP, but it can be used to request family and private life visa, assuming can prove have enough funds etc. So there aren't citizenship rights.
We don't have the same adoption rights, so would be adopting as individuals rather than a couple, but happy to get married if adopting a child together!
The other big attraction is that to end the civil partnership, just send a letter to authorities. No divorce, no delays, no reason or agreement needed. So it can be helpful with assets or, if you don't care about assets, it can save a divorce! It's a stepping stone to marriage or long term committed relationship for many.
Most are ended so that can get married, it's about 50-50 heterosexual to same sex.
It's a really good system to be honest, it doesn't replace marriage on significant things such as citizenship and adoption, but it works well for couples living together who may want to get married in future, or want to commit/benefit from some of the rights given to a cohabiting couple without worrying about divorce drama.