Is marrying a girl previously engaged to you, but rejected by your parents, a sin punishable in this life and the hereafter, knowing that the purpose is to preserve one's chastity?

We advise you to try and convince your parents to allow you to marry the girl, by involving some relatives or respected figures. If they do not agree and insist on forbidding it, then do not disobey them and seek someone else, out of obedience to them. However, if refraining from marriage would cause you harm, or if their prevention is stubborn and without valid reason, then their obedience is not obligatory upon you, and you would not be disobedient by disagreeing with them. You must always honor them and treat them with kindness, and avoid harming them with words or actions.