“Marriage has always had to be legal”
Wrong. States didn’t begin licensing marriage until the 19th century.
It isn’t wrong at all.
A marriage had to be legal, to be legal, if it wasn’t legal, then you were not married according to the law.
You didn’t have to be legally married in the past, and you don’t now, if you so choose.
But if you want your marriage to be legal, then it must be recognized as such.
While America only goes back to “the 19th century” licenses go back to the 1300s, and when it wasn’t a license, it was a bann, or something else, the license isn’t the requirement, even today Americans can marry without a license, yet still have a legal marriage.
If you don’t care if your marriage is legal or not, then don’t worry about any such things, just make up your own definition.