It's not quite this.
In medieval England, legally, all an English couple needed to do in order to be married was to declare that they were. 'I, John, take you, Jane, to be my wedded wife' and vice versa.
It was convenient if there were witnesses, as otherwise it was very tricky to prove (you get court cases where it's clear both parties had had sex and one or other of them had been lying about marriage). But it wasn't strictly legally necessary.
Legal quibbles come up when people argue over the form of words (eg. 'I never said we were married; I said I'd love to marry her if I could but I can't!' or whatever.
But there was no obligation to involve the Church or the State in contracting a marriage. This is, in effect, common law marriage.
I don't think it's stupid that people don't know what legal rights they have; I do think it's something (along with relationship health and financial awareness) that ought to be taught more explicitly. If you have never been told any of these things, and everyone around you has been stating them as obvious facts all your life, it isn't surprising if you assume it's all true.
IME there are quite a lot of legal situations people don't know about - simply because they don't encounter them.