1) Supply is already everywhere, you won't be increasing supply probably by more than 5-10% upon legalization.
2) The real gateway drugs are alcohol and nicotine. Using this logic is an immediate right to criminalize both of them. Both are far more addictive, kill WAY more people (marijuana is impossible to OD from), and alcohol is much more of an intoxicant.
3) The marijuana costs of the drug war are staggering, accounting for roughly 50% of all arrests, with a total budget of over 50 billion a year at all levels.
4) Legalizing marijuana would seperate the soft drugs from the hard drugs and people wouldn't be pushed to purchase cocaine, crack, meth, heroin from their local gov't authorized marijuana distributor.
5) Legalizing marijuana would reduce the risk of lacings till close to 0 with marijuana.