Actually my parents are the same way. The only reason my dad stopped smoking it is because he got something unknowingly laced one time. When I got caught my mum was "disappointed," and my dad was fine with it, as long as I don't get caught, and use it responsibly. Anyway long story short, the only way to get them not paranoid is to either explain in detail that dealers lose profit from lacing things (they lose customers, usually it costs money to lace something with, and finally it takes up a lot of their time to lace bud with something.) or tell them you want to grow it. They can't really argue with that, just say you'll keep it to a 1-5 plant minimum, far below dealing laws, and say that there's no possible way it can be laced straight from the ground.