To me Indiana's final rejection of society for love seems like an indictment of society and the rules and regulations that govern it. Throughout the story we're presented with the ways in which society's notions of acceptable love are corrosive and damaging to both virtue and innocence. Both Indiana's husband Colonel Delmare and her would-be seducer Raymon de Ramière are men society accepts and rewards with wealth and prestige, despite the fact that both are abusive and thoughtless partners. Delmare is controlling with a violent temper and a disregard for the desires of those around him, especially Indiana; de Ramière is fickle, cruel, and superficial. Indiana searches for love within the confines of society and fails at every turn; she is abused and humiliated for her efforts.
It is only when she at last escapes society that she once again finds the will to live—and only outside of society's confines that she recognizes the merit in Sir Ralph, who has been there all along but never until now received much of her notice. Maybe Indiana suggests that men and women—and love—exist best and most naturally together away from the pressures and rules of society.