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To say that its not a mortal sin and not  a gateway to the demonic is ridiculous.  You may laugh, say I'm crazy, but I had demonic experience with it. And when people say that its not dangerous I think "how ridiculous." Because I am still living with that experience that forever changed my life. That was 15 years ago. I can hear them now, "it doesn't happen to everyone and you exaggerate". Whatever. You have no idea how evil it is.
It amazing what some Traditional Catholics think!

I am sorry for your bad experience.
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I don't say it is never a dangerous thing.  For fourteen years we lived in a location where we were alienated because we did not use marijuana, so I do know something about its dangers and its effects. I am not naive about these things. And I don't wear the label Traditional Catholic.
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Some medications have drastic effects too, so if your doctor puts you on antidepressants and you take them you are committing a mortal sin? Maybe you are unwise to follow doctor's prescriptions, but do you sin by taking them. For some people anti-depressants could be a gateway to demonic influences.
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It may well be a mortal sin for you if you were to use it, because you now have the knowledge of how it affects you. But to make a general rule for all...
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So how much did you have for how long did you take it and what other influences were there?

I am against the use for reasons of those who drive, using equipment.  You can measure pot, if one is pulled over.  Call it DUI, but still can not be measured.

Blood/breath levels cannot be measured, but driving while impaired doesn't rely on blood/breath tests.  You can be impaired on prescription meds too.  Police can do a field test for impairment.  Any substance must be used responsibly.  I think education is the key.  We never used to consider scripts as "impairing", but now with education people realize they can be impaired on meds their own doc prescribes.
Although I don't have any stats to back me up, I suspect we had a huge problem with public impairment after prohibition was repealed.  Finally through education (most) people have learned not to drink and drive.  I suspect the same will be true with MJ. Don't smoke and drive.
I don't think it is the MJ that is the problem, but people's lack of responsibility when using it.  I keep coming back to the alcohol analogy because I think it is the closest there is.  Use responsibly.
It's not that I think MJ is a good thing, I don't.  But neither do I think drinking or smoking are good things.  However, people are going to use recreational substances so let's just hold them accountable for their actions if they cross the line legally (not just demonizing them for using).
I realize I'm probably the lone voice on this board to defend the responsible use of MJ, but for me it all comes down to personal accountability for one's actions.