I can't really remember. I was aware of this fetish at a really young age, and I was a little bit of confused, because the others didn't seem to notice anything special in these things which were arousing me a lot. Later I found people like me in the internet, and that was a really big relief for me. I was not alone.
Wow! I can relate to that in a big way. I know for a fact that I was aware of my paraphilia in the 3rd grade because an incident is forever burned in my memory. A bunch of us kids were in the locker room changing for gym class engaging in the usual boyish banter. One of the guys asked the group if they ever noticed becoming erect. One guy reported that this happens to him when he sees a photo of a naked woman. The whole group voiced affirmation that this was, indeed, the most common cause of this bizzaere physiologocal phenomenon.
In my youthful naivete, I immediately chimed in that seeing a muscular woman causes this, too. Instead of the validation I expected, I was greeted with chirping crickets.
I was like Ben Stein in "Ferris Beuller's Day Off" - anyone? anyone? After that I kept my mouth shut about what I like.
It was so liberating to find on-line communities like this. There's more of us out there than I ever could have immagined.