https://en.wikipedia.org/wiki/Clandestine_Marriages_Act_1753
https://www.citizensadvice.org.uk/family/living-together-mar...
https://www.familylives.org.uk/advice/your-family/relationsh...
Edit: also a related term, parental responsibility...
https://www.bma.org.uk/media/1840/bma-parental-responsibilit...
Edit2: also you can assign each other as "Next of Kin".
https://www.royalfree.nhs.uk/patients-visitors/advice-and-su...
Edit3: also note, married couples don't automatically have "Lasting power of attorney" which grants partners a say in their medical care, and being "Next of Kin" is not an equivalent.
https://www.ageuk.org.uk/information-advice/money-legal/lega...
So... don't get married, sign your name on the birth certificate, and fill in that NHS card
https://www.gotocourt.com.au/family-law/de-facto-relationshi...
"The courts can make an order for the division of any property that you own together or separately. They may also order a split of any superannuation, or that one party pay spousal maintenance.
The net asset pool will include anything acquired before, during, or after separation. It does not matter whether the property was owned jointly or individually. When determining a property settlement, the court evaluates the types of contributions – financial and non-financial – made by either person, as well as their future needs."
Edit: this applies to AU "de facto relationship" vs UK "common law marriage" terms... it might be different when discussing different countries.
Canada too.
Every US state recognizes common law marriage from other jurisdictions, and (because of federal Cobstitutional rulings) does so without regard to the gender of participants.
There can be complications if one party claims to be common-law married and the other does not, especially during divorces.
https://globalnews.ca/news/7327501/couple-no-home-no-kids-sp...
These laws are awful and need to be changed.