Sorry, but thinking it's "harmless" is NOT logical thinking. It's agreeing with your own position, perhaps, but not necessarily logical.
"Logical thinking," IMO, acknowledges the harm that a particular substance can cause while ALSO acknowledging the benefits that that substance can provide. In order to do this effectively, one must do research before deciding whether to use that substance, weigh the pros and cons, and then, once having used it, evaluate the effects of that substance on one's own particular body, mind, and spirit.
(if you wanted the short answer, stop reading now)
To more fully answer your question, I believe that adult marijuana use should be decriminalized in my country (USA), because I believe strongly that I have a natural right to put in my own body what I see fit. I also believe that marijuana definitely does not belong on the US government's Schedule I, which is defined as:
A) The drug or other substance has a high potential for abuse.
(B) The drug or other substance has no currently accepted medical use in treatment in the United States.
(C) There is a lack of accepted safety for use of the drug or other substance under medical supervision.
Marijuana most certainly does have accepted medical uses, such as the treatment of nausea, anorexia, and glaucoma - they may not be accepted by the DEA, but they are by medical practitioners.
That said, smoking is the main method of ingestion of marijuana, and smoking anything
can damage one's lungs and increase the risk of certain cancers. Studies show that short-term memory loss (reversible in adults but less so in those whose brains are still developing) is indeed a side effect, and enough attacks of "the munchies" have certainly impacted the waistlines of some stoners I have known. So it's not "harmless" either.
I have found that most things in life, drugs included, are neither exclusively "good" nor "bad," and like you, I tend to judge who I will engage with intellectually based on the extent to which they reject black-and-white thinking.