(Seriously asking a person whether they have started a game in multiplayer rather than single player when trying to play coop is borderline insulting)
No, it isn't. People have had the settings wrong. It isn't very common, but sometimes people do miss obvious things, and it is trivial to check. Regardless, after a couple email I did add a line after that question acknowledging that people would more than likely have already tried a new multiplayer game at some point.
