Readers of Dorothy Allison's Bastard Out of Carolina are shocked and appalled by the protagonist's mother's betrayal of her daughter when she leaves with her husband, knowing that he sexually and physically abused her daughter.  Readers generally consider her reprehensible for choosing 
her husband over her daughter in this case.  The audience of Tennessee Williams's Streetcar Named Desire, however, is much more understanding of Stella's decision to believe Stanley rather than Blanche.  Stanley is her husband, Blanche merely her sister.  Even as we don't approve of Stella's choice to remain with Stanley, our disapproval is more out of concern for her well-being, staying with a man whom we know to be a rapist, than for Blanche's: we are worried about, not angry with Stella for her choice.  Perhaps this is in part because the self-involved, deceptive and manipulative Blanche is not so appealing, but I would argue, too, that viewers' sympathy for Stella's decision has as much to do with our culture's value of the marriage relationship over any platonic relationship other  than one's relationship with one's children.

In the novel at hand, Toni Morrison has Sula break the generally accepted amendment to the commandment thou shalt not commit adultery: in particular, thou shalt not sleep with thy best friend's husband or lover. And she develops Sula into a not-so likable character, making it easy  for her reader to condemn this woman for betraying her friend.  But Morrison, typically for her, is setting us up.  This novel is no All about Eve.  In that movie, we observe the machinations of the duplicitous and 
manipulative Anne Baxter character (Eve) as she uses the Bette Davis character  (Margo) to worm her way into the theatre and then not only usurps Margo's role in the theatre but also tries to steal her husband and destroy her reputation.  There is no question of what we are supposed to think about Eve's self-centered, back-stabbing ambition--and no problem sympathizing with Margo, who cannot imagine that a woman whom she has helped and whom she considers a friend would betray her.  In stark contrast, Morrison's Sula did not set out to hurt or take anything from her friend; and in further contrast, Morrison's Nel never even gave Sula the benefit of the doubt, did not for years, in fact ask her why she did it.  And when Nel finally does ask, Morrison hits the reader with Sula's responding questions:  "If we were such good friends, how come you couldn't get over it?" (145) and "How  you know . . . [a]bout who was good?" (146).  While it appears to Nel that Sula has chosen an afternoon's pleasure with Nel's husband over her  friendship with Nel, actually it is Nel who chooses Jude over Sula:  apparently out of loyalty to her marriage to this philandering man she cuts off her friendship with Sula, and she maintains the estrangement from Sula even after Jude is long gone.