Marijuana a Gateway Drug?

I recently got into a brief debate with someone over whether drugs, and in particular marijuana, should be legal or not (the discussion arose after I made the (somewhat) tongue-in-cheek comment that if Detroit wants to survive, it should become the next Sin City).

Anyways, this person said he was against legalizing marijuana because it is a gateway drug. Now the Gateway Drug Theory as I understand it, when it comes to marijuana, is more or less defunct as a legitimate scientific theory. At the very least, it has been acknowledged by most in the scientific community that tobacco and alcohol are much "stronger" gateway drugs than marijuana.

This gateway argument doesn't hold for two primary reasons. The first is the age old rule of statistics: correlation does not equal causation. In other words, substitute the word "milk" for "marijuana" and you have the same exact argument. I believe that substance use or abuse is much more a revealer of who someone is and the conditions that affect their life as oppossed to being causes of a "bad lifestyle." Of course drug use/abuse can contribute to a downward spiral, but I'm unconvinced it is primarily due to drug use or is a chicken/egg problem.

Secondly, I think this argument reveals a great deal of confirmation bias within the proponent. And that confirmation bias is that a lot of people, for whatever reason, hate and are freaked out by weed. Even though marijuana is far less dangerous than tobacco and alcohol, there is a bizarre taboo that surrounds marijuana that has only been perpetuated by decades of federal taxpayer-funded propoganda campaigns. Why people have such an irrational fear of marijuana relative to alcohol and tobacco is beyond me, but if you find yourself discussing this issue with someone and they refuse to acknowledge that tobacco and alcohol are just as gateway-relevant as marijuana and thus should be made illegal as well, than this is most likely the reason why.

If anyone is familiar with scientific papers that find marijuana a more potent gateway drug than alcohol and tobacco, please forward on to me. My searches have been futile so far.