True but they also need to consent with their parents.
Plus also mentality. Their conscience should bear to them they're married. For if they do sex without faith, now that is sin ; the sin of fornicating. Great sin that.
But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin. Romans 14:23
Once married, comes adultery. Another eternal sin till death. If your conscience bears you witness you're married, so are you. But if it bears that you're sinning, fornication it is.
Be wise where you are. Married or fornicating. If married, with no divorce unless she commits sexual immorality.