So when a state legalizes marijuana, do more people start using it? Do kids start using it more? These are real concerns. Fortunately, data is starting to come in and it looks really reassuring. No, more people don't start using it and no, more kids don't start using it. Here's an article that links to a survey from CO, AK, OR, and WA.
It's probable that more use leads to legalization, not the other way around. So we can relax.