Gateways

Marijuana is not a Gateway Drug. Marijuana Prohibition creates a Gateway Crime.
Caffeine gets you high. That doesn't mean it's a gateway to heroin which also gets you high. But when you buy a joint, you are entering the black market & engaging in the criminal underworld where all sorts of stuff is bought & sold.
Going to the grocery & picking up some Coke does not introduce anybody to the whole criminal underworld thing.