How to convince my parents a nose piercing isn't so bad?

I'm going to turn 18 soon, so i can just go ahead and get it pierced myself, but I still don't want my parents to look at me like I'm going around behind their back and begin a "bad kid". I don't even want a large hoop or anything, just a really small silver stud. Why do they think it's such a bad thing? I don't know if it's because they think it looks trashy (which, I'm far from. i like to dress cly, and nose rings can look girly, like kelly clarkson's) or because they don't want me to grow up (they still dont allow me to wear make-up, dye hair, etc.). How should I go about telling them?