The answer depends on how and whom you ask, and what you mean by “bisexual.”
The Kinsey Group, who interviewed thousands of men and women in the 1940s, found that half of men had been sexually attracted to another man and one-third had experienced orgasm through physical contact with another man. Very few of these men considered themselves gay; most had also experienced sex with women. Many surveys since have produced similar numbers.
The most recent survey comes from the CDC, who interviewed more than 12,000 men and women about sexual behaviors in 2002.
Nine out of ten men told the CDC that they were heterosexual and only attracted to women. But among the 10% who did not identify as straight, only one quarter considered themselves gay, and even fewer said they were attracted exclusively to men. Most men who were not straight considered themselves bisexual or “something else.”
Six percent of men said they had experienced oral or anal sex with another man, and half of those men still considered themselves straight, while another quarter considered themselves bisexual or “something else.” Only one quarter of men who had had intercourse with another man considered themselves gay, and even most gay men had also had sex with women.
What accounts for the differences between the CDC and the Kinsey research? The CDC was primarily interested in diseases, so it asked only about anal and oral intercourse. The CDC also skipped over men who lived in institutional settings, such as dormitories, military barracks, or jails, where sexual contact between men is more common. Also, any survey which forces you to label yourself as gay, straight, or bisexual is going to miss the much larger group of heteroflexible men, who are bi-curious but likely to call themselves straight, especially in a face-to-face survey.
Note that this research does not reflect rates of bisexual desire and behavior around the world. Research in South Asia, for example, has found rates of anal sex between men as high as 20 to 50%, even though virtually all of them are married or will marry. Even in the US, rates of bisexuality were higher among women and among black and Latino men, according to the CDC survey.