I think that there is a difference between being wrong and being guilty.
Marion had to choose between two evil deeds, and made her choice.
Probably she was wrong in hoping that Robin could accept it, but for judging deeply her behaviour we should know her feelings and her intentions.
I want to say that the same deed can be guilty or not , depending on the intentions of the person who did it.

Anyway, I feel that the story has an happy end, since Robin was not worth being married - not since he left her, but since he abused and offended her.

Ciao
Emanuela