In several countries, and absolutely historically, it’s actually been a government-classified relationship status.

It just means people involved in a romantic and sexual relationship who are unwilling or unable to legally marry for whatever reason (because they are already married, because they aren’t legally qualified, such as one member is from a slave caste, etc)

https://en.m.wikipedia.org/wiki/Concubinage

By definition of concubine, sure, but consensual is a far cry for most. I’m trying to remember the exact quote, but I’ll paraphrase it here: ‘In slave-owning societies, the vast majority of concubines were slaves.’ https://archive.org/details/historicaldictio0000klei for the source.