"It doesn't matter what argument I make in support of usage, she will always counter with its legality."

I had a friend like that with the same argument. Of course he had no problems with the legal issues of driving while intoxicated, etc..

Some people equate legality with morality because they don't want to spend the time constructing their own moral code, forgetting that slavery was once legal and anyone helping an escaped slave was a felon.

But again, with my friend, legality was only his argument. What it all boiled down to was he saw himself as better class of people than those 'stoners'.