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For what purpose do they need the THC for?

Marijuana (rightly) has a stigma attached to it that makes it seem trashy. The smell is enough to render it much more disagreeable than alcohol or tobacco.
Weak argument as someone can use a vapouriser completely bypassing the smell and health defects of smoke.

Look at the members of society who abuse marijuana and what type of people they are. By smoking marijuana, you would be casting your lots with them.
 Strawman. Plenty of successful people drink alcohol, and plenty of 'unsuccessful' people drink alcohol. By driving a car you are like bad drivers? Or are you like good drivers. "Look at people who abuse alcohol"

There is more to the argument than simply the ability or inability for marijuana to cause the complete loss of reason.
Overuse of alcohol can also cause the loss of reason. Why are you using that marijuana usage = getting high?

1) Stigma attached makes it completely scandalous and in most traditional circles will result in complete irreparable loss of reputation.
No one said anything about public use or around trads

2) Scientific studies have proven beyond any doubt that marijuana use destroys brain matter, causes insomnia, contributes to the development of schizophrenia in adulthood, and makes you gαy, etc, etc.
Some of these are just outright lies (making you gαy ) . Also people can develop the same issues from alcohol. Schizophrenia for example has to do with those who have dispositions for it...

3) I have friends who had smoked marijuana daily before becoming traditional catholic. They are unanimous in agreeing that marijuana use is 100% a mortal sin.
Sounds like someone who lacked discipline and now projects that onto others. "I drank alcohol everyday that means drinking alcohol is 100% a mortal sin"

4) Not an academic argument, but look at Rome (built by alcoholics) vs Portland (built by stoners)
modern alcohol is very different from past alcohol, pesticides, cleaning Chemicals used on the barrels and other factors. Where the romans drunk or just had some alcohol?

5) The only reason a sheltered traditional catholic child would ever try marijuana is because they were given the impetus to as a result of some gαy rebellious desire, which presumes that they know that it is stigmatized and bad.
"The only reason a sheltered traditional catholic child would ever try ALCOHOL/CIGARS is because they were given the impetus to as a result of some gαy rebellious desire, which presumes that they know that it is stigmatized and bad."

6) Its gαy and retarded
You already had ridiculous points earlier but this is clearly nonsense (aka brainwashing cope, there is no logic and reason here)

7) Just drink alcohol like your European ancestors have for 6000 years
A certain Catholic saint used marijuana for health reasons perhaps you've heard of Saint Hildegard? Also you do realise that many plants were used by people in the past before Jєωιѕн companies banned them because of (((reasons)))?

8) "Ohhh brrruuhhhh I feel so chilll right now brruuhhh" it makes you talk and act like a retard. Whereas alcohol amplifies charisma in the charismatic, industriousness in the inquisitive, and geniality in the sociable.
you are comparing someone high to someone who had a little of alcohol. If you have too much alcohol you also become retarded. Also alcohol doesn't effect everyone in the same way.