It's just a statement of simple legal fact, though. Just because your social arrangements appear identical to those of a married couple, it doesn't make you in any way legally married as it's the ceremony and registration that count in law and not the relationship. Marriage is foremost a legal contract.
A 6'6" 17-year-old is taller than most adults but still legally a minor; a 2'8" 18-year-old is shorter than almost all but the very youngest children, but is legally an adult - as height, although a common factor in differentiating the two groups, is not the legally-accepted criterion - only actual age counts..
Stirling Moss was a top-flight professional racing driver for many years and nobody ever doubted his excellent advanced driving abilities; however, a the time, he had never actually passed a driving test (I don't know if he ever did), so he didn't have a driving licence and, legally speaking, was not a qualified driver.
In law, everybody is a non-driver until they pass a driving test; everybody is a minor until they've lived for 18 years; everybody is single until they marry.
These are just legal facts and in no way a value-judgement of the strength of your relationship.
You can both absolutely hate each other - maybe never loved and only ever married for tax purposes - but if you've been through a legally-recognised ceremony and haven't since had it annulled or divorced or been widowed, you're still legally married. If you've been devotedly in love and faithful to your partner for 80 years, but haven't been through the ceremony, legally you're still no more married than a child is.