Most studies of the biology of homosexuality are based on the theory that almost everyone is either almost exclusively heterosexual or almost exclusively homosexual. According to this theory, there are no true bisexuals. This theory is based on the 1948 Kinsey Report, which asked people the extent to which they were attracted to members of the same or the other sex. Genetic studies have used people only at the extreme end points of the Kinsey scale in order to have as strict as possible a definition of hetero and homosexuality. The Kinsey studies have not been repeated and might show a different distribution now than when data were collected in the 1940's, a period when most people were reluctant to report bisexuality.
Historical evidence flies in the face of the bimodal theory.
It was usually considered deviant, however, not to marry and have children. Many therapists still proceed on this assumption, but they no longer try to convert bisexuals and homosexuals into heterosexuals. Evidence from literature, history, and popular culture also suggests that bisexuality may be a common phenomenon. At the very least, current themes are politically charged. Some wish to believe that homosexuals are born, not made.