In Jane Eyre, marriage is about a combination of three things: compatibility, passion, and ethics. A marriage only works between like-minded individuals with similar attitudes and outlooks on life. Inequalities of class background or financial situation are surmountable, but characters who marry for wealth or status are doomed. But a marriage has to have more than common ground – it has to have passion. Characters who try to match themselves up based on rational criteria, the ones who are doing nineteenth-century-style online dating, sin against their own natures, as do characters who try to claim that marriage and love are the same thing.
Rochester’s relationship with Bertha Mason is no longer really a marriage, and he should not be considered ethically bound to her as a husband.
Rochester’s attempt to convince Jane to elope with him while Bertha is still living results from his redefinition of marriage in terms of interpersonal sympathy rather than a social contract.