Even if it is illegal, it is possible to procure marijuana from sources from which one is certain that no harm has come to any individuals through drug cartels and the like. One can grow or purchase organically grown marijuana with no additives and no history of coming from questionable sources.
The illegality issue is a somewhat minor one, as being “caught” with less than an ounce of marijuana results in a simple fine in most states in the U.S. It is a misdemeanor that is no more serious legally speaking than a large speeding ticket (and unlike with speeding, no-one’s life is put in danger with marijuana use).
Marijuana can be vaporized and the harmful effects of smoke virtually nullified as well.
Also, to say that marijuana makes you stupid is a ridiculous generalization. Just because a lot of stupid people use marijuana, does not mean marijuana makes one stupid. Go to any college campus and a LARGE percentage of the professors there use marijuana, not to excess, but on a fairly regular basis, and their IQs tend to range in the 140-160s.
Any “sin” (if you want to call it that) from responsible, moderate marijuana use under the above criteria would be venial at best, and that only for breaking a minor law, which, if we are honest with ourselves, most of us do nearly every day by failing to come to a complete stop at a stop sign or exceeding the speed limit, which can actually put the lives of others in danger.