My story, in brief:
I was born this way.
I played doctor with little girls up to about 11 years old, I daydreamed about high school girls, and my first girlfriend in college happened when I was 18.
My mother recognized it early, but went into denial phase and tried to scold the tomboy out of me.
It didn't really work.
So now I'm a slightly dominating, lipstick wearing non-straight person with a slew of dildos. Thank my mother for my feminine accent.
I can do men, but it isn't the same.
There is a neat episode of 'Queer as Folk' where one of the group males tries to be straight...watch it. I'm sure its at your local video store.