My assessment is based on an assumption, but one that is unconnected with gender. I am assuming that Marion's own code of morality included a belief that sleeping with someone other than her life partner was wrong. On that basis, Marion violated her own morality, albeit for the noblest of reasons. If the tables had been turned, I would have concluded the same thing about Robin.
My own personal code of morality makes no allowance for judging the actions of others, but to the core of my being I hope that if ever I were presented with a choice like Marion's I would not violate that personal morality, whatever the cost. You asked
If i could save the world by lieing down with the devil-- what would be the right thing to do? Value myself more than the world? -- Is that moral? and I would never presume to answer that question for anyone other than myself. For myself, lying down with the devil is out of the question, under any circumstances.
In addition, I do not think that Marion's actions made her a slut. Assuming that she had violated her own code of ethics, she would need support, loyalty and consolation, not abuse or condemnation. Her noble motives also must be taken into account. I know of people who have betrayed their own morality under immense pressure and who were helped to recover from doing so by the compassion and understanding of those around them.
I am a lazy sod, and I like living my life by a simple set of values. I have no interest in expecting others to live by my values, or in condemning them for not doing so. I also know that I have never yet been placed in the sort of situation under discussion, where personal morality would be sorely tried. I am, however, proud to know several people who have had their personal morality grievously tested, and who passed those tests. I can but hope that I would do as they did.
