Years ago my (now ex) wife and my friends would tell venial sins on road trips. It started out as a bit of a joke, but as we got going the level of intimacy grew quickly.
Like everything dealing with morality and what-not, there's no clear division. From a traditionally religious standpoint, there isn't really much of a division either. Christians would say that Jesus died for all sins, not just a handful of the most interesting ones.
I would say that the line between venial and mortal was a line drawn by man to feel something.