I don't remember exactly when, but I do know that it was prepubescent, so I didn't associate anything sexual with it until later. The oldest and most vivid recollection I have is of two roller derby queens going toe-to-toe during a live telecast (black and white TV, of course), but I doubt that this was the start. I still really enjoy looking at the old black and white roller derby clips that you can find on Youtube; those women really went at each other when they "fought".
Realizing that this was sexual thing didn't occur until I was a young teenager and happened to be watching an episode of the Avengers one evening. Emma Peel and this hot villaness built up tension throughout the entire story and finally, during the climatic showdown, were slugging it out behing a movie screen ----- and then it just happened. I'm still in love with Diana Rigg.