Humans as a species are neither heterosexual, nor monogamous by nature. Modern societies and organized religions have tried to defy human nature making both the norm, leaving many human beings confused. The term bisexual paints far too broad of a brush to cover the actual interests of people that are not 100% heterosexual, or 100% gay, which is only about 6% of people. I'd say that maybe about the same amount that are truly bisexual. If you are truly bisexual, this means that you are both emotionally and physically attracted equally to both sexes. Most people are not.
A majority of humans have at least masturbated with a friend/cousin/sibling of the same sex. We also know that this behavior often leads to more mutual things. This is usually done under the banner of "helping each other out" and/or practicing to be ready for the opposite sex, not considering it bi or gay. As we mature, most end up as heterosexually minded and bury the fact that this sort of thing ever happened. Very few go on to be gay, lesbian or truly bisexual.
Since roughly only 10% fall into being hardwired as totally straight/gay/bi, the rest of us choose our sexualities. Because of the stigmas, most choose to be either straight, or gay, Both sides tend to label anyone that chooses bisexual as being confused, or in denial of being gay/lesbian.
There are so many different ranges of desires of the majority of us that are not 100% straight, or 100% gay. Most of us are usually much more attracted to one or the other and choose that as our identities. Some only ever fantasize about the same sex, but would never act on it. Some enjoy recreational same sex play to get off, but have zero interest in more than just getting each other off. Many that identify as gay/lesbian have similar ideas about the same sex.
The moral of the essay is that the labels that society and religion forces upon us are outdated and inaccurate. Humans are sexual beings. It would be a much better world if we did away with labels and stopped judging what other consenting adults do in private.