I feel the question asked from the outset is not being framed properly, because on it's face it would seem illogical that simply smoking something is a sin. If one digs deeper, realizing that anything that turns us away from God constitutes a sin, the use of marijuana is certainly a sin.
I used marijuana for a few years, thinking that there was nothing wrong with it and only after I had stopped did I realize the detriment it caused. Setting aside the obvious negative physical health effects, it, along with other drugs, skews one's sense of reality. In doing so, it makes that person more susceptible to the evils encountered on a daily basis - and potentially others. For example, I encountered dreams of future (insignificant) events, I felt inspired in my writing and allowed my ego to take over. Anyone who is an avid online video gamer would notice many people consumed by those games who use drugs like marijuana to enhance their experience. Now I don't mean to say that everyone will have those exact experiences, but those are the types of things that drug use can cause.
In each of our spiritual struggles, we attempt to battle our passions, carrying our own cross to hopefully gain salvation. How can we expect to do so when we make it easier for our passions and for temptations to take hold of us? Why would we counteract our forward spiritual steps by weakening ourselves to the powers of the evil one?
In conclusion, insofar as smoking marijuana leads to sin it certainly is a sin itself. Just as we shouldn't act to facilitate another's sin, we should take that same approach not facilitate our own sin through means harmless at first sight.
That's how I have come to understand it anyway...
The best thing: Speak to your spiritual father, because trying to read over 10 pages of forum posts will only confuse the issue even more.