When I was about 12 and my sister was 10, we lived with our father in a small three room apartment in Texas. The bathroom was across the small living room from the bedroom so when my sister and her friend were around I would go in and take a shower. After my shower I would come out in a towel wrapped around my waist. The first time I did this I left it loose so about half way across the room it fell. The feeling of standing totally naked in front of them was amazing and I was hooked. I think they enjoyed it too as soon each of them returned the favor. I have been an avid voyeur and a cautious exhibitionist ever since.
I still enjoy allowing an women a peek of my cock and normally wear shorts without underwear which facilitates flashing when safe.
There are of course additional stories from my growing up years and I will add those as time permits. Enjoy.