No hate mail, but I would have to disagree on this point. Once upon a time, I would have agreed with you hands down. Age and experience, it seems, have conspired against me. I don't disagree that most people have a preference, but I've met quite a few people in my day that seem to defy falling into one category or another.
I would argue that Kinsey basically had it right - that essentially everyone is 'bi' to some degree - most less so than others. On a scale of 1 to 6 most people tend to gravitate to one end of the scale or another. Cultural taboos tend to keep them there. But personal experiences, attitudes and circumstances can allow one to drift in one direction or another.
Keep in mind that I'm talking about sexual orientation - not sexual activity. Anyone can have a sexual experience that is not in keeping with their orientation. But even sexual orientation can drift a bit when taboos are shattered by personal experience and the relaxing of attitudes.
When I was younger, I tended to believe that sexual orientation was 'fixed' at birth - most likely genetically. As I've grown older and experienced more, I tend to believe that it's not so much 'fixed' as it is 'fluid', but with a predisposition - very similar to the way a person is born with a talent, but can develop that talent into a skill.
While most people do tend to gravitate in one direction or another, there are quite a few that lie basically in the middle - perhaps with a slight preference in one direction or another - but for all intents and purposes they are 'in the middle' or 'bi.'
Don't get me wrong. Calling ones self 'bi' is still a popular form of denial. But that fact that many gay men use it as a way to deny their sexuality doesn't mean that others don't fall squarely into that category. Personally, I tend to believe that there really is no such thing as 'straight' or 'gay' - only shades of 'bi'.