I don't know either. I just do..
Maybe an imprint from playing around at 9-10 with the college-aged guy next door. His cock was huge compared to mine..and probably big, anyway. All I remember is that it was mostly fun, exciting, and pleasurable..except that he often came by getting me to bend over, mounting me, and rubbing his balls up and down on my back. In hindsight, that was weird.
The ladyboy thing, for me, is about cock and female together. I'm not turned on by any trans that looks male which is why I mainly prefer the most 'feminine', hairless asians.