He always refers to them as a stepping stone or a first step. When I told him that it isn't dangerous or addictive, he said that somehow, they still are, just less, and that people have a strong tendency to go into harder drugs, and that's their demise.
I'm not sure what to believe, but I'm still curious about its effects.
I think the illegality of marijuana is what causes it to become a "stepping stone". You need access to the drug, so you need (or needed, 20 years ago) to find someone to sell it to you. This person was the drug dealer, or a customer of one. The drug dealer had a little of everything -- merchandising = profit -- so if you heard about another drug, like cocaine or crack, you could ask about that, and get it.
I think it's different today, because medical mj doesn't connect you with the drug underground.
On the other hand, doctors overprescribing vicodin, a pretty addictive drug, has become the gateway to heroin. It's because the drugs are similar.
He's right that it's historically been a "gateway" to other drugs. That's something we need to recognize and deal with (as I wrote in my piece), but not reason enough to ban it either.