Oregon's Measure 91 and the prohibitionist mentality
The first thing I noticed about the "anti" arguments is how many of them talk about children. I have a general rule -- whenever a politician starts ranting about "the children", keep a firm grip on your rights, because he's revving up a pretext to snatch some of them from you. Under Measure 91, of course, marijuana sales to children (to anyone under 21, in fact) would remain illegal, just as the sale of alcohol to minors is illegal now. Some minors will certainly get access to marijuana despite the law, as they now sometimes get alcohol despite the law -- but minors are getting marijuana right now as well. "Illegal" does not equate to "impossible".
Ever notice how stores that sell alcohol never try to sell to kids? Most of them vigorously enforce ID checks. That's because they have a profitable legal business selling to adults, a business they could lose (along with other consequences) if they sold to minors. In contrast, since drugs are illegal across the board, the pusher has no reason not to sell to anyone who's got the cash. He has nothing to lose. Regulation works better than prohibition.
There is a vast range of things which are forbidden when children are involved but legal for adults, because even if those activities are unwise or somewhat harmful, adults cannot be denied such choices in a free society. If we banned adults from having everything that might be bad for children, on the grounds that allowing them means children might get access to them, we would no longer have a free society (and none of those things would actually cease to exist, anyway).
Much is made in the "anti" arguments of the sale in Colorado of marijuana-laced candy and similar products for users who prefer not to smoke the stuff. In a few cases, children have eaten these by mistake and gotten sick. This, however, is an argument for clear labeling of such products and responsible handling of them in the home, not for prohibition. Some alcoholic drinks are hard to tell from ordinary fruit juice by taste or appearance, and if they were not labeled clearly and if a parent carelessly left them around, a child could be poisoned by them, too. This is not an argument for banning alcohol across the board.
The "anti" arguments also raise the issue of accidents due to driving or operating machinery while "high". This could happen, but it would still be illegal, just as drunk driving is illegal even while alcohol more generally is legal. Drunk driving is a major problem, of course, but no sane person takes this as an argument for returning to Prohibition. And broad laws against marijuana do not prevent cases of driving while on drugs even now, just as Prohibition didn't eradicate drunk driving.
There are a few people who just can't handle drugs or alcohol safely. That does not mean everyone should be banned from having those things, any more than the fact that a few people can't handle cars or guns safely means that I and 200 million other adults should be prohibited from having them.
Is the drug experience degrading and disgusting? There's certainly a case to be made, and as someone who had a severe drinking problem for more than 20 years of my life, I'm well aware of the degradation of addiction. But such reactions are ultimately subjective. No doubt many fundies are honestly revolted by the thought of homosexuality. A person is likely to feel repulsed by any indulgence which is not to his own taste. Such feelings are not a basis for limiting the choices of others.
It is possible, of course, that some negative effects of marijuana on society might become somewhat more common if it were legal. But to do a true cost-benefit analysis, we have to look a the huge harmful effects of prohibition which would disappear if it were legal -- the mass-scale incarceration of people who have done no harm to anyone, the criminalization of a whole industry (with loss of potential tax revenues), the harm done by products whose quality cannot be regulated since they are illegal, the erosion of respect for law in general because of the existence of a law which is widely flouted and obviously irrational.
We need to free ourselves from the error that when we vote on whether something should be legal, we are voting on whether it is allowed to exist. Some things exist whether we make them legal or not -- all the law can do is change the conditions under which they exist. Alcohol prohibition did not make alcohol consumption stop. It did make alcohol impossible to regulate, put alcohol production and distribution in the hands of criminal gangs (who thus gained great wealth and power), and made the entire business far more dangerous, violent, and sleazy. The present-day criminalization of drugs and prostitution has the same effect. Prohibition of porn or guns will have the same effect if we are ever foolish enough to try it.
Drugs can even, with some effort, be obtained in prison. Think about that. A prison is a far more tightly-controlled environment than a whole country can ever be. No law could ever actually keep drugs out of a whole country, not even if it made that country as miserable as a prison.
Some of the arguments focus on specific weaknesses in this particular measure, suggesting that the opponents might favor some initiative to legalize marijuana, just not this particular one. Of course, no new law has ever in history been perfect -- when reform is necessary, you enact the best laws you can, then modify them as time reveals the need. This kind of objection reminds me of those who pointed to the inevitable flaws in Obamacare as grounds for rejecting it, which would have meant sticking with the nightmarish old system with its tens of millions of uninsured, its "pre-existing conditions", its staggering costs and mediocre health outcomes compared to other rich countries, forever -- because of course no actual proposed reform could ever have been good enough. Luckily, we passed it in the knowledge that we could adjust and improve it over time as flaws came to light. Measure 91 will be better than the present madness of prohibition, even if it is not perfect -- and we can improve it and modify it as needed.