Many people have a mistaken idea that marriage is a ceremony. That by going before a priest, or other official, having him say some words and then getting a document makes a man and woman husband and wife. The reality is that a ceremony is only a part of it.
Sexual intercourse is marriage. The first time a man and woman join, they become one flesh, and they are married before God. No ceremony is needed for this part to take place.
So in other words, this statement may seem strange, but they have entered into a marriage outside of wedlock. They have become married without covenants. The wedding ceremony is not where the joining of man and woman takes place. It is in the marriage which is the actual joining of the man and woman.
I am in no way downplaying a wedding ceremony. I believe it is proper for the man and woman to first enter into covenants with each other before joining. I think by doing so, the resulting marriage is stronger so long as both persons keep their vows.
My real point is that in the world today, you have young men going around marrying young women, and women entering into those marriages, without any covenants. They then part their seperate ways and sleep with someone else, immediately committing adultery. They then act like some day they will settle down and get "married" without realizing that they already are.
Again, I know this idea is controversial, but it is scripturally backed. This is why there is such emphasis in the scriptures on chastity and virtue. If a young girl is not a virgin, then she is already married. Likewise for the man. If you sleep with someone who is already 'married, you are committing adultery. If you marry some who is already married, you are committing adultery and causing them to do so as well. Don't go around marrying people you don't intend to spend a lifetime with. Save yourself for for that person God intends you to spend your life with. Treat it seriously.