There's also the argument that MJ's a gateway drug because of correlation - "you don't see many heroin users who didn't start with marijuana". My general reaction is "Look at all those heroin users who avoided marijuana because it's illegal and dangerous! (Oh there aren't any?)" (Actually there probably are some, people who got addicted to prescription opiates they started using for medical reasons, and switched to heroin because it's cheaper and because they can't get enough legally and weren't getting good medical support for getting out of the addiction.)
I did know one guy for whom marijuana actually was a gateway drug - first time he got high, in high school, he decided that it was good stuff and they'd been lying to him about all the reefer madness stuff, and figured he should see what else they'd been lying to him about.