The common wisdom holds that people are born gay or straight. Bisexuality is thought of as a rare phenomenon that proves the rule. Those who identify as bisexual are viewed with suspicion and commonly thought of as “really” just gay.
There probably are genetic contributions to sexuality, and this becomes most apparent at the extremes, but it is misleading to say that everyone’s sexual tastes are determined at birth and limited to one sex or the other. In fact, just the opposite is true: most of us are born with the potential to be attracted to both sexes.
Most natural traits exist on a spectrum and can be described by the normal distribution “bell” curve. In the case of sexuality, culture forces the bell curve to one side:
Homosexuality is stigmatized, marriage and procreation are encouraged, and we are taught that everyone is either straight or gay. These and other forces conspire to push the vast majority of men and women into identifying as straight and suppressing their feelings for the same sex. Only those who are predisposed to be exclusively homosexual end up identifying as gay or lesbian (and surveys show that to be around 3% of the population).
Our culture also holds several myths about bisexuality.
Another way to think about sexuality is to envision overlapping categories, with many of us falling somewhere in-between the traditional categories of straight, gay, and bi.
You can read about each of these and several other sexual types in individual posts:
Of the more than 8,000 people who have taken my Flexuality Test, about one-third turned out to be ambisexual, and another third were heteroflexible. True straights made up less than 1 in 10 of the total, even though most people who have taken the test identify themselves as “heterosexual.”