Under a very old tradition, that I was raised under, if two have sex, they are “One in the Flesh” permanently. Under that tradition, the marriage was not defined by an Oath, License, or Ceremony, but had occurred when two had sex, the rest was just window dressing.
If they are permanently faithful to each other, I have no problem with this.
See I’ve always wondered how that viewpoint would look upon a virginal victim of rape or molestation. Will they forever more in the eyes of God be married to their rapist even though they go on to get married and be faithful to their spouse till death?
I’ve known people who’ve had long term shackups - living together with someone for 5 years then living together within a couple of months with someone else for several years before finally getting married.
And they still call it their first marriage.