How do I convince my parents I am responsible enough for...?

A webcam. I have been wanting a webcam for about 3 years now. You know, the average reason anyone wants a webcam, to talk to their friends (on skype). My stepdad being the so called funny man (he thinks) he is says that he fears I will go naked and want to show myself to boys. Now I don't have a record AT ALL of being promiscuous so I don't know why he is comparing me to some of those s l u t s out there. My mom is saying no because my stepdad is saying no. I need real help. No sarcastic answers. Could you tell me what to say. And FYI my stepdad is German and Polish and Dutch and all those things, so he is VERY VERY VERY VERY stubborn.