Well my dilemma has nothing to do with me living under their roof. I'm just wanting them to understand it, and why I choose to use it and all that bullshit they were fed when they were younger isn't true. I want them to be able to respect the decision that I made to smoke pot, and not look down their nose at me for doing so.
Then when the subject comes up, remind them that it obviously isn't something they want you to honestly discuss and tell them that you'd appreciate it if you could change the subject. If they're unwilling to respect the decisions that you as an adult make for yourself, then they don't need to know about those decisions.
If it truly is an adult-to-adult relationship then they can't really bitch about you deciding not to tell them every single detail of your life. I'm sure there's things that your parents do that they'd prefer not to tell you about... well, the door swings both ways, you know?