I think that the terms "aunt" and "uncle" don't really rely so much on gender as they do on the role they play in a child's life after the child reaches the age of 5. An aunt's role is to disapprove of you, and an uncle's role is to tease you and tell you outrageous lies that make you paranoid for the rest of your childhood (my uncle Mark STILL has my nose...)
I haven't shaved my legs in a while, and so yesterday while I was wearing capris, I managed to convince my 5 year old nephew, Nekoda (who was crawling around under the dining room table and licking people,) that my legs were feral and that they would bite him if he got too close. He listened with wide eyes, then tried to poke me in the leg. I made a snarling noise and he shrieked and ran away.
I think I'm an uncle.