The problem with your argument is you ignore one of the driving forces behind the ban on prohormones: the potential impact of the drugs on children.
While alcohol, tobacco, and marijuana can all harm teenagers, the occasional use (or even the development of a habit that lasts a few years) is unlikely to cause permanant damage to the teen's health. However, 1 cycle of steroids, especially without ancillary medications or PCT, can cause permanent, irreversable damage to a teen's body (premature closing of growth plates, permanent damage to HPTA). For adults, tobacco and alcohol are potentially more harmful. But to teens, a strong steroid cycle could do more damage than a year of smoking a pack a day. Restricting the legalized steroids to adults won't work either. I drank more in high school than I did in college, and I started smoking at 16 (I no longer smoke, however). Furthermore, most of us here cycle responsibly. However, many teenagers would adhere to the "500mgs per week is great, 1500mgs per week will be better" mentality. And most teenagers think they are invincible. I know that when I was 17 I thought I would live forever. Why do you think that the military sends people off to war at that age? No matter how much evidence we can put out there that steroids can be safe for adults, the potential consequences for teenagers alone are more than enough to prevent legalization from ever happening.
As for marijuana, it will never be legal in the recreational sense. For medical purposes, before it approves it, the FDA will require a delivery system other than smoking. First, smoking itself is harmful, and second, it is impossible to accurately measure the dosage. Since I know you can get high from eating marijuana, maybe a THC pill.
Look, I don't mean to be pissing in your Wheaties, and if you want to start a movement, go ahead, and I will be happy to help point people in the direction of the website. But it's never going to happen. Period. No legislator is going to risk his career going against such overwhelming public opinion.
/karp