supafly, just a couple comments:
If part of marijuana supposedly being a "gateway" drug has to do with its being illegal, wouldn't legalization make it less of a gateway drug?
And along the same lines, legalization / decriminalization (I know you favor the latter) would also have the benefit of not making young users "criminals" who might then be more likely to commit other crimes -- real crimes
Also, I don't think anybody who favors legalization would make it legal to smoke and drive. Would it happen? Sure -- just like it happens now, and happens with drinkers and other drug users. But let's not conflate the two.
How many of those marijuana users had first used cigarettes and/or alcohol?
There is a purpose in comparing weed with alcohol and cigarettes -- I can think of at least two, actually. First, it's to demonstrate that weed is no more harmful, and probably less harmful, than these two drugs that are socially acceptable. Second, it's to demonstrate the inconsistency in our laws on adult usage of these drugs.
it isn't a matter of moving on from cigarettes to stronger cigarettes or alcohol to stronger alcohol -- if that were the case, the analogy would be moving from weaker weed to stronger weed. The gateway hypothesis is that marijuana users move on from marijuana -- a weaker "drug" -- to stronger drugs. That's exactly what the argument is regarding tobacco and alcohol -- that use of those is the actual gateway, first to weed and then (in some cases) to harder drugs.
I've never looked at stats, but I know anecdotally that every person I know who smoked weed first tried alcohol and/or tobacco, myself included. I also know anecdotally that most people I know who smoked weed never used anything harder (and most of those who did use something harder just experimented, as opposed to becoming regular users).