I would not know about over there, but I know for the Benelux (and Sweden has something exactly if not very close to it) they have something called a "samenlevingscontract", which literally means "living together contract". It can contain what you want, as much as you want making it like a marriage and more (quite similar anyway), or as little as you want. You cannot just draw one up yourself, it has to be done with lawyers and through a notary, who has to have it on file, if not most judges will declare it invalid when it comes to issues with taxes or government (inheritance, paternity, estates and taxes, etc). In Sweden the term is exactly the same, they call it "sambo" for short (forgot the long word for it), forgot the details for a sambo though but it was quite similar to the Benelux.
A marriage you could import, like from Morocco or Turkey for example (99% of marriages with multiple wives were just that), could be between many wives and a man, as long as you say "religion, racism!" and they just accepted it. And then the government caught on, a dude with 4 wives in 4 homes on welfare as he had a gf in his own house, nope. So now a marriage is strictly between 2 people. So polygamists use the samenlevingscontract but just have a lot more people sign it. And because such a contract can contain exactly what a registered partnership entails (save for a few details) there you go.
Important to note that a registered partnership is a marriage where you do not have to say yes but still have to bring witnesses, and where you do not automatically were the daddy like in marriage, and they changed that to always the daddy in 2014 anyway. There are a few more things, like in case of divorce and then death in marriage all your stuff goes to your ex, the end, which does not fly in registered partnerships. All in all the differences are minor, and it takes 2 minutes and 2 signatures and saying yes to each other and you are now husband and wife instead of partners, the differences are that minor.
You can still make a legal entity to protect assets, and it probably could be made to work like a marriage somewhat, but rules are a bitch to make that work, NV is anonymous as the N "nameless" part implies which bla bla, conclusion will not work, not even mentioning the minimum cashflow, and minimum sold and paid for shares, €40K minimum, personal responsiblity with BV since they are all flex versions now, could try Ltd. but tax service will be eating out of your ass everyday and you can count on extra risk with that, have to start it up in London and then go to Commerce Chamber to transfer shit, lot of work and money and it would not serve its purpose well or at all. Forget fancy stuff and weird shenanigans, just a few contracts and done.
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