As far as I know these two were united by God before the foundation of the earth and in God's eyes they have always been married. Do you actually think that a judge or preacher or rabbi or Muslim cleric or Hindu priest declaring you married makes it so?
So what is the definition of "fornication" then if it's not unmarried sex.
And we are repeatedly warned to "abstain" from it and "repent" from it in the Bible.
By your logic....there's no need of marriage, the foundation of families. We can just go around saying...."we're married in the eyes of God" and live common law.
Come on....I'm not attacking the young couple. You asked the question and I answered it.
"Fornication" IS a sin and we're ALL sinners. If not of that very human sin, then of some other. But thank God for His Grace which forgives our sins by the blood of Jesus Christ.