Because it does in other jurisdictions. Australian law is very different on this matter with ‘de facto’ relationship being a legal thing, same as marriage. There are loads of Australians in the UK and vice versa, so it stands to reason people take their experiences with them, and share them with friends.
Say a couple go to Australia on a working holiday and end up settling for a few years, living together. Australian law recognises that as a legal relationship. But if they move back to the UK, it’s suddenly not. But marriage, which is also a legal relationship, would be treated as being valid in the UK. It’s understandably confusing.
Australia and the UK have very similar legal systems, so this is a huge difference. I enjoy mumsnet for the UK focus, fair enough that’s the assumption of a person’s location. But people make general statements based on their jurisdiction, without stating where they are located, and those statements, made with great authority, do no universally apply across the readership of mumsnet.
Basically, buyer beware. Everyone’s situation is different!