Okay, here I go again . . .
In 1948 Alfred Kinsey and his team postulated that there are seven degrees of sexual orientation, from 'exclusively heterosexual' (level 0) to 'exclusively homosexual (level 6), where an individual from levels 1-5 fit describes just how hermaphroditic they are. (They preferred that term to 'bi-sexual.') There is also a further category 'X' that alludes to people who are asexual.
The original studies, Sexual Behavior in the Human Male (1948) and Sexual Behavior in the Human Female (1953) are great readings, if you like science and statistics. (Guilty!) The Wikipedia articles on 'Kinsey Scale' and 'Kinsey Reports' aren't bad.
Of course, since then there have been many, many studies that have furthered the research (or muddied the waters, depending on your point of view.) But most of them that I've studied simply come to much the same conclusion - Bisexuality is a part of human nature and should be considered sexual orientation as much as Heterosexuality and Homosexuality.