Is part of it that Marlow-ness provides no way of a graceful backing down from a position once assumed? Karen can't go back from wanting to marry Edwin once she's announced it and defended it to the family (though she might have just to her mother), just as she can't go back on it to Edwin. I'm trying to think of any occasion on which someone climbs down from a principled stand, and I can't - Lawrie over Lois being my closest.