"Gateway drug" is a term many different meanings as to render it virtually meaningless.
As far as I know there is nothing about marijuana that specifically makes you more likely to want to or use other, "harder" drugs. For people who seek new sensations and the thrill of the forbidden (as young adults often do) pot is naturally the first illegal drug they encounter, being the most prevalent in most areas.
From there, I'd say two factors come into play. One is that getting involved in using MJ regularly is likely to put you in contact with use, sell or can find you other drugs. Second, having experimented with pot and not found it to be the terrible bogeyman that the authorities have made it out to be, one is likely to discount the other (often valid) warnings that they have heard.
My personal opinion is that we'd do a lot better trying to educate people to respect their minds and bodies and use substances responsibly. Look at booze, for example. A drink now and then, or even an occasional bender, is not necessarily a bad thing. But drinking and driving, or becoming an alcoholic is. For whatever reasons, though, and I can think of a few, many young people use alcohol completely irresponsibly. Similarly with pot. Coke can be used occasionally and responsibly. Not sure about heroin - certainly most people have to desire to stick a needle in their arm just for fun.
Anyway, I think prohibitions of various substances miss the point: Why do so many people wish to lose themselves in a drug-induced state for so muc of their time? We don't seem to have put very much time or energy into answering that question.
"Illegal" being the operative word. Seems to me that if an individual is willing to break the law to get one drug, they'd be willing to break the law to get another.
Who's more likely to "gateway" to robbing liquor stores: a teen who works for a living or a teen who shoplifts? Now, you can certainly make the case that not all shoplifters go on to rob liquor stores. Or even that some shoplifters are actually well-to-do.
But that, of course, misses the point. The point being who's more likely.
Keeping teens away from illegal soft drugs (marijuana, Ecstacy, nitrous, GHB, etc.) helps to keep them away from illegal hard drugs.
You do have some valid points there. I don't know why so many people feel the need to be high or drunk to have a good time. Personally I can't stand wasting my time talking to a drunk. I hate the fights and tears. My father owned bars and I learned early to avoid drunks, mainly him. I have seen the ugliness come out in drunks. You made a point about heroin use and needles. You don't need a needle to use and become addicted to it, just a straw or a dollar bill. Heroin has become a cheap drug and in my town there are many teens using it. We have many people dying from it and ruining their lives. Such a shame.