Poison of an ally, love of a Bene Gesserit

Frankly I never bought it that Lady Jessica's entire decision to defy her order was based on simple love for her duke and knowledge that he wanted a son. Lady Jessica is too smart and the decision to pivotal for that. I have taken liberties with some pre-Dune speculations but this is what would make sense to me. I own nothing except the randomly named Desho Lek.