Are all girls bisexual?

[Reasonable]

^^
good point - terms like Heterosexual, etc. are coined from attempts and views based on clear definitions (which makes sense from a classification point of view). But in reality anything to do with sexuality and sexual urges tends to be much more complex rather than clear cut.

And of course social context throws a huge overlay on people's own basic makeup and tendencies.

I have to say though, the old 'all women are Bi' always raises a smile for me, because 99% of the time it'll be a bloke bringing it up or 'interested' in such a possibility.