Lately, there has been many atheists trying to promote the legalization of marijuana as soon as it was publicised that it didn't have many (or any) physical problem associated with it. Whether it's safe to use or not, I don't think legalizing in public is something we should do.
Firstly, if it is being smoked, then everyone around you will also be doing the same (second hand smoking). People with delicate lungs (eg: emphysema) or sensitive senses (eg: Autism) may end up having problems with others smoking this in public. If marijuana were to become legalized, then it should be eaten or drunk instead of smoked. This is technically the same issue as cigarette smoking in public
Another problem associated with this, is that even though marijuana is 'ok' for you, the smoke itself and not the chemicals in the smoke is what is bad for you. Let me explain; our lungs have evolved to breathe in ~70% nitrogen and ~20% oxygen. Breathing in smoke is essentially ~35% nitrogen, ~10%oxygen and ~55%smoke. If this were to be breathed in for long periods of times, and on a regular basis will cause lung problems. When people say that marijuana isn't bad for you, they mean specifically marijuana, because consuming it in any other way won't give you lung problems.
Even though using marijuana may be physically 'ok' to use, there are still side effects that come along with using it. According to http://www.schizophrenia.com/prevention/streetdrugs.html, 8%-13% of schizophrenia cases are linked to marijuana. Even though this may not seem like very much, people who regularly use marijuana increase their chances of schizophrenia by 600%. This means that if everyone in Australia (22 million) became a regular smoker of marijuana, then 145200 people would end up with schizophrenia. BUT not everyone does smokes it, so the numbers will be smaller. If marijuana became legal, then we are allowing (statistically speaking) 145200 to 'obtain' schizophrenia. The population of the USA is ~300 million. If marijuana was legal and everyone became a regular user, then 1980000 people (statistically speaking) will get schizophrenia. If the numbers are lower, say 1/5 Americans become regular users, then 396000 people (statistically speaking) will get schizophrenia. (The maths for this is simple, 66/1000 users according to http://www.schizophrenia.com/prevention/streetdrugs.html have schizophrenia from marijuana use; 66/1000x(population) and then you have your answer).
All in all, the main purpose of this is to say that if marijuana is legalized, then it shouldn't (legally) be smoked, rather it should be consumed some other way.