What should a girl do who lost her virginity after a relationship with a married man, whose marriage she was unaware of, and her family refuses to let her marry him?

Islam does not condone a relationship between a man and a woman before marriage. You have committed a great error and sin by engaging in indecency (fahisha). Sincere repentance is accepted by Allah, and its conditions are: immediately ceasing the sin and staying away from this man, regretting what happened, and making a sincere resolve not to return to it. It is advised to increase good deeds and choose righteous company. You must conceal yourself and not tell anyone what happened. Do not inform your future husband about the loss of virginity due to adultery; you can use allusive speech (tawriya). You are not permitted to accept a marriage proposal from the man with whom you committed indecency unless he repents sincerely and your family approves of him.