Can someone tell me what this muscle is called?
 

Thanks to my good friend Wikipedia, I was able to locate this for you.

Iliopsoas - Wikipedia, the free encyclopedia

It is a combination of 3 muscles, psoas major, psoas minor, and the iliacus. They're usually just referred to as the inner hip muscles, and they are important for standing, walking, and running.

(Also, chicks have those muscles too; the formation of muscle in a guy's body and a girl's are just different due to differing body structure in that region. :) )