Sigmund Freud posited that everyone is inherently bi-sexual many years ago.
See here:
INNATE BI-SEXUALITY
He claims that all humans are born bisexual but through psychological development (which includes both external and internal factors) most become monosexual while the bisexuality remains in a latent state.
The gay pundit ( who debated William F. Buckley ) Gore Vidal also believed in this.