Viliam:

I got the impression that the anti-euthanasia guy is mostly repeating one argument -- that you can always remove pain, therefore the argument of killing people to prevent them from feeling unnecessary pain is always false.

...From what I have seen, hospitals for terminally ill patients are more or less torture chambers.

But sure, if you had argued that we should view marijuana use as approaching torture in badness, or even that the opposition takes this view, that would be responsive.

Now the Catholic Church does in fact seem to oppose marijuana across the board. As an outsider I find this ludicrous (will they treat it more like wine once someone claims it can become the breath of Christ?) but certainly I would not have downvoted you had you presented your claim as an explanation of the (specifically Catholic) opposition and not implied that I should share your objection.