Generally-speaking, fornication is sex before marriage. That would be like marrying someone and finding out she lied about being sexually pure before your marriage, then divorcing her.

1 Corinthians 7:28 But and if thou marry, thou hast not sinned; and if a virgin marry, she hath not sinned. Nevertheless such shall have trouble in the flesh: but I spare you.

This is not talking about REmarriage, but about marrying in the first place.