Great thread. Late to the party, and this is more of a strange story than a conspiracy, but always thought the story of teleporting Spanish soldier Gil Perez was very interesting..
I think the gateway drug myth had more to do with someone trying something illegal for the first time. Underage drinking was different because it was legal at a certain age. Marijuana on the other hand has always been illegal. Maybe you were more prone to try other drugs once you crossed that...