Everyone isn't bisexual really. Not all bisexuals, either. We don't think everyone who is sexually attracted to more than one gender needs to identify as bisexual.
We agree that people choose labels and identities for a wide variety of reasons. When we define bisexuality we're not doing it to tell people they must call themselves bisexual.
Some people don't use "the b word" because they want to draw attention away from the genders of their partners. Others feel that "more than one gender" doesn't do enough to highlight the wide range of gender.
If that is you, please don't think we're saying you have to change and identify as bisexual. We think of 'bisexual' as a sexual orientation as something that encompasses and includes such identities as queer, pansexual, bi-curious, questioning, and indeed 'bisexual identified'. Although you might not as individuals use the word as an identity label, it's the category of sexual orientation (as opposed to homosexual and heterosexual) that we're talking about when we talk about the bisexual population of the UK, or the bisexual sector of LGBT.
After all, you don't have to identify as a bisexual to be discriminated against for being bisexual.
How would we prefer people to identify? With whatever label they feel best suits, fits, captures, sums up, or empowers them.
How do we want people to define bisexual as an identity? The same way we define bisexual as a sexual orientation - attraction to more than one gender. We've got one word for both orientation and identity, let's have one definition - but this doesn’t mean there should be only one identity open to people.
There are also people who define bisexual differently, and then either don't define as bisexual themselves or don't want their friends to do so. They define bisexual as being all about having a lot of sex, or about definitely not being attracted to trans people, or about having to have a male and female partner at the same time, or about being 50/50. Very few people who identify as bisexual add these things on to their own definition of the word, and we think it's time this baggage was recognised as such.
We don't think anyone who is only attracted to men and women should stop calling themselves bisexual, either, we just want to make it clear that a wider definition includes more people. You don't have to be attracted to "more than two"!
It's been said that "perfection is achieved not when there is nothing left to add, but when there is nothing left to take away". We don't think our definition is perfect, but we do feel that taking a lot of things away from the definition of bisexual is necessary to allow the widest number of people to see themselves in it, and to make it clear that when we talk about "the bisexual population" (as opposed to "the bisexual-identified population") we aren't excluding people who do not yet identify as bisexual, or who have considered the identity and chosen another.