From the New York Times:
Some people are attracted to women; some are attracted to men. And some, if Sigmund Freud, Dr. Alfred Kinsey and millions of self-described bisexuals are to be believed, are drawn to both sexes. But a new study casts doubt on whether true bisexuality exists, at least in men.
The study, by a team of psychologists in Chicago and Toronto, lends support to those who have long been skeptical that bisexuality is a distinct and stable sexual orientation. People who claim bisexuality, according to these critics, are usually homosexual, but are ambivalent about their homosexuality or simply closeted. “You’re either gay, straight or lying,” as some gay men have put it. In the new study, a team of psychologists directly measured genital arousal patterns in response to images of men and women. The psychologists found that men who identified themselves as bisexual were in fact exclusively aroused by either one sex or the other, usually by other men.
More here.