The difference between adultery and fornication is simply that adultery is sexual misbehavior in a case where one or both parties are married to another, while fornication is sexual misbehavior between two unmarried people.
Because adultery occurs where at least one marriage exists, it is regarded by the law as a violation of the legal contract of marriage.
Fornication, however, violates no legal contract. For this reason, people tend to regard more lightly than adultery. Since fornication involves two consenting individuals, it is unlikely for most people to even consider that a third party has been offended. As a result no proper repentance takes place.
Even when we consider just the lustfulness of the act itself, we find it easy to argue that after all it is natural and biological, and besides, the older moral codes have changed. But again it is the grievous wrong done to another that God wants to uncover. And in spite of what people think, He considers fornication to be as wrong as adultery.
The fornicator will have to repent as deeply as the adulterer if he/she is to have peace with God.
