How do I get the girls to quit calling me gnat?

I used to always make jokes about myself having a large penis until one day a girl said ok prove it in front of a bunch of other girls. I chickened out because I actually have a very small penis (less 2 inches not hard) now they gave me the nickname gnat until I can prove them other wise they said. I know I will never lie about my size again but how do I get the girls to quit calling me gnat?