I don't think there is anything inherently sinful or evil about smoking pot.
But <<putting on my old crank hat>> a few things:
1) The last time I smoked pot was in the 1980s, maybe it was the early 90s, I don't know. I didn't smoke a lot -- just a few times. The pot you could get was weak compared to the stuff I smell all around me now. Even with that weak stuff. ... my observation in college was that a lot of the people I saw smoking a lot got LAZY from it. Gross generalization, but my thoughts at the time were that it saps your ambition. I can't imagine what that strong stuff is doing to a generation now. I suspect it's not good, in the aggregate.
2) But my big problem today isn't that I care whether anyone smokes. It's none of my business what you do to your body. ... even though I do think it's probably not very smart, long-term healthwise; still, that is your choice and I shouldn't have anything to do with it. What bugs me is smelling it everywhere I go. I find it obnoxious. If you want to smoke, smoke. Decency (if not the law) should dictate that you do it away from others and don't make it so I have to walk through a block of your likely harmful second-hand smoke when I am on the street going somewhere.