*w* Surprisingly enough, I can answer this question in full, though I feel like I've answered it before, but I cannot remember.
I recall, being six years old, and I was playing hide and seek with my older sister in the backyard. Now my sister is cruel...very, cruel. And I was hiding behind a swing like the dumb six year old I was. (One of the larger swings built for two, like the gliders I believe? The love seats that swing.) and she pulled it forward and shoved it back and I went flying into a fence. Given I was alright, my mother spanked the crap out of her, and even though I hate F/f of any sort, that was what sparked it.
From there it grew to a Mickey Mouse Christmas, to an episode of Pokemon on my Jigglypuff Pop VHS, then back to AnimeBrats, and HandPrince I think it was? This goes WAY back for me. I remember reading spanking fics online, Dragon Ball Z, Full House, Harry Potter, all I could find and it took me a long time to be able to do it when my parents were home.
Weak self-esteem and all, y'know.

Nervousness was also a horrid factor. People are always TERRIFIED they'll be shunned if their parents find out, but I'm one of those people who believe "Mommy will love me no matter what". So. -shrugs- x) I'm no longer afraid to share my fetish.