It is extremely difficult to estimate the extent of Bisexuality because it depends very much on how the term is defined. For example, it could be people who identify themselves as Bisexual (in which case the estimate might be rather small), or it could be all people who have ever had an aesthetic, romantic or sexual attraction to more than one gender (in which case the estimate might be rather large).

In the 1940s, Alfred Kinsey put forward his famous scale of sexual orientation, finding that many people did not fall simply at either end of the spectrum (Heterosexual or Homosexual).

In terms of the proportion of people who are actively involved in UK bisexual communities, over two hundred people participate in the annual BiCon event - just a small proportion of the Bisexual-identified people who are involved in local and on-line communities. Fears of Biphobia may prevent some people from being 'out' about their Bisexuality.

Bisexual people are not necessarily attracted equally to both sexes. Because Bisexuality is often an ambiguous position between Homosexuality and Heterosexuality, those who identify, or are identified, as Bisexuals form a Heterogeneous group. "My primary preference is men," adds August. "I pretty much 'dabble' in my Bisexuality....sometimes I have the need, sometimes I don't."

Others view Bisexuality as more ambiguous. Some people who might be classified by others as Bisexual on the basis of their sexual behavior and /or self-identify primarily as Homosexual. Equally, otherwise Heterosexual people who engage in occasional Homosexual behavior could be considered Bisexual, but may not identify as such. For some who believe that sexuality is a distinctly defined aspect of the character, this ambiguity is problematic. On the other hand, some believe that the majority of people contain aspects of Homosexuality and Heterosexuality, but that the intensities of these can vary from person to person. Some people who engage in Bisexual behavior may be supportive of Homosexual people, but still self-identify as Heterosexual; others may consider any labels irrelevant to their positions and situations.

For example, I for one identify as Heterosexual; however I have kissed men many times, attend open play parties AND mens-only play parties and don't particularly mind certain male-male intimate/flirty contact with myself and others, and am very supportive of our LGBT community; however the desire to engage in sexual activity with another man has not been a burning desire per se...so what am I? Am I straight? Am I Bi? Am I confused? I'd love to hear your answers on that!

Pansexual, omnisexual, anthrosexual, and pomosexual (postmodern sexuality) are substitute terms that, rather than referring to both or "Bi" sex attraction, refer to all or "omni" sex attraction and are used mainly by those who wish to express acceptance of all sex possibilities, including transgender and intersex people, not just two.

Pansexuality sometimes includes an attraction for less mainstream sexual activities, such as BDSM. Some people who might otherwise identify as pansexual or omnisexual choose to self-identify as Bisexual because the term Bisexual is more widely known, and because they see it as an important term in identity politics.

Bi-permissive describes someone who does not actively seek out sexual relations with a given sex, but is open to them. Such a person may self-identify as Heterosexual or Homosexual and engage predominantly in sexual acts with individuals of the corresponding sex, and might be rated 1 or 5 on Kinsey’s scale.

Near-synonyms include heteroflexible and homoflexible. Ambisexual indicates a primarily indiscriminate attraction to either sex. A person who self-identifies as ambisexual might be attracted with equal intensity on physical, emotional, intellectual, and spiritual levels to partner(s) regardless of sex or gender presentation while upholding selectivity standards in other areas. Some might experience equally intense attractions that could be triggered by sex- or gender-specific traits in the given partner or partners. A person with this orientation might fall in the 3 category on Kinsey's scale, as would some who subscribe to the 2 or 4 rating (although some individuals in these latter categories consider themselves bi-permissive).

Bi-curious has several distinct and sometimes contradictory meanings. It is commonly found in personal ads from those who identify as Heterosexual, but are interested in Homosexual "experimentation." Such people are commonly suspected—not necessarily correctly—of being Homosexuals or Bisexuals in denial of their Homosexuality. It can also be used to describe someone as being passively Bi, Bi-permissive, or open to indirect Bisexual contact.

Says Brian Phillips of Meiguolangren Zai Taibel: An American Werewolf In Taipei, "I really don't think it's part of a 'boom in Lesbianism' in and of itself. I would say that's more of a subset of the 'Bisexual chic' phenomenon of Britney kissing Madonna kissing Christina or Girls Gone Wild videos or girls on the dance floor kissing one another...sure, as experimentation, but also to get the guys excited! Yes, there is a so-called 'Lesbian boom' in popular culture and the like, but it is part of a wider experimentation in sexuality, particularly the search for alternatives."

He goes on to say, "Certainly, experimenting with Lesbian behaviors and feelings is real and does seem to be growing, particularly among the young where same sex experimentation by women has always been fairly high (see anyone from Kinsey's original report to Masters and Johnson to Hite). However, experimentation is not Lesbianism, it's experimentation! The interesting change is that Bisexual and Lesbian experimentation are becoming more socially acceptable in media and within the youth cultures. Openly caressing and kissing on a dance floor is very different from the guilt-ridden girls experimenting behind closed doors praying they weren't 'freaks' and that they wouldn't get caught."

Trisexual (sometimes trysexual) is either an extension of, or a pun on Bisexual. In its more serious usage, it indicates an interest in Transgender persons in addition to cissexual men and women. In its more humorous usage, it refers to someone who will try any sexual experience. It is used in the song "La Vie Boheme" in the Broadway musical Rent. The term was coined by porn actress Robin Byrd.