I know that I could find a supplier of marijuana if I wanted. I'd just need to walk 100m down the street and ask the school kids hanging around in the park. Easy.

But I also know that I would be risking various legal and social consequences. A conviction for possession of drugs would affect my employment options, it would affect my travel options (visa forms often ask about such convictions), it may cause issues in the future if I ever got into a custody battle for my kids.

Given all of that, I have no plans to walk down the street and buy some weed from my friends' kids.

But I do have a collection of cigars in my house which I enjoy on occasion.

If it were possible to buy marijuana in the same way I can buy cigars, with the same legality, for a comparable price, then I might do so.

Now, I don't really see why society should care if I were to substitute some of my tobacco consumption with marijuana consumption, but there's a reasonable probability that this would be one of the outcomes if marijuana were totally legalised.