I NEVER understood why people were so up in arms about Tosh's actions in GBG. First off, Mary was HOT and charming and seductive and it'd take one really perceptive individual to see beyond all of that. Second, as you said, it hadn't been the best week ever. Heck, it hadn't been the best YEAR. Suzie had only died a few weeks ealier, then everything went wrong with Ianto and Lisa, and finally, the horror that was Countrycide. No one would be in the best state of mind at that point (look at Gwen's behavior), which is exactly why Mary chose that moment to swoop in. She was no dummy and neither was Tosh. Tosh was vulnerable and lonely and Mary took advantage.