There's also some interesting stuff in the book about the Mutual Admiration Society discussing Dorothy's friendship with Muriel St. Clare Byrne's partner Marjorie Barber -- that book does give the sense that Byrne was not always particularly kind about juggling her multiple ongoing relationships, and that Barber's struggles with being cast as the 'wifely' domestic partner who waited at home while Byrne lived the more glamorous and intellectual life resonated with Sayers in her own arguments with Heterosexuality And Gender. So it's possible that she was just drawing a line, although I certainly wouldn't rule out an Oxford crush either.