This is exactly what's wrong with the "gateway drug" argument. Where does the line of "gateway" start?
I don't remember the first time I stealthily nabbed one of dad's beers and drank it. Let's say I was 13. And I probably nicked a cigarette from him around the same time. I do remember the first time I smoked a joint, behind the bowling alley with some older kids in the Drama Club, I was 15. I think this is a pretty typical experience, I wasn't some crazy dirtbag kid or anything, an honors student from a middle-class suburban home.
As of now, over 20 years since then, while I enjoy weed form time to time, I've never had even the slightest desire to try any other "illegal drugs". But for this example, let's imagine that when I was 18, I got into heroin.
How could you define weed as the "gateway drug"? It was the third in the list of mind-altering drugs. I have never met a pot smoker who hadn't tried alcohol first, and usually nicotine too.
One could try to argue that weed was the first link in that chain classified as "illegal" drugs, but beer and cigarettes are illegal for teenagers. You could try and argue that beer and cigarettes are weak enough that people can use them for purposes other than getting a buzz, but again, not the case with teenagers.
I can respect "straightedge" people who speak out against drugs. But if you actively smoke cigarettes or drink caffeinated coffee or enjoy an alcoholic beverage - regardless of if you're taking it to extremes or staying moderate - you're using "drugs" that alter your mental state. To criticize someone else's choice of intoxicant is sheer hypocrisy, especially if you attempt the "gateway drug" argument.