Well, my prob is that I find males plain old ugly. If I can't get into ticklee's role in my head, /m just doesn't do it for me. That's why I'm ok with f/m tickle stories, and even those I prefer from the first perspective.
To me, watching a female tickled is a beautiful sight. I like to appreciate the beauty of the one tickled, her smile, her shape, and the state she is in. Seeing a guy tickled (and all guys are just ugly to me) kinda ruins the picture. Still, imagining myself getting tickled by a girl... now that's a completely different thing.
But, then again, I was never a guy for watching... gimme action and let me 'on 'em!