There was a study which was widely cited, including in Sex at Dawn, which showed that women are more likely to be sexually fluid than men. They showed videos of straight porn, gay porn, lesbian porn, and monkey porn to various different groups of people, and it was observed that straight men only showed physical arousal when watching straight or lesbian porn, while gay men only became aroused while watching gay male porn; women, on the other hand, became physically aroused at all the types of porn they were shown, even if they didn’t realize or admit they were aroused.
I think there are definitely people who are 99-100% gay or 99-100% straight, but as we move into a future where sexual orientation doesn’t matter as much, our culture may get a little better at allowing people to skip self-labeling and just like who they like… which I think will result in a lot of open bisexuality.
People who say “Everyone is secretly bisexual” kind of bug me, though. Mainly because they themselves are always bisexual. It’s exactly like a straight guy saying he doesn’t think any man is actually gay, because he himself can’t empathize with what that would feel like.