Bisexual folks, and specifically bi men, exist and there is some great scientific research being done in this area. In the past, some research studies have cast doubt on the level of arousal bi men feel for both sexes - they found more attraction to men than women - but they didn't get the recruiting right and that affected the outcome [duh!]:
Past research not finding bisexual genital arousal patterns among bisexual men may have been affected by recruitment techniques. For example, bisexual men in those studies needed only to identify as bisexual and to self-report bisexual attractions (e.g., Rieger et al., 2005). Thus, the bisexual samples of previous studies may have been populated by men who had never or rarely behaved bisexually and perhaps identified as bisexual for reasons other than strong arousal to both sexes.... Additionally, bisexual participants in past studies were partly or exclusively recruited from the gay community. For example, the bisexual sample of Tollison et al. (1979) was recruited from a university gay student union. Thus, past studies may have unintentionally oversampled bisexual-identified men with homosexual arousal patterns.
Dan Savage has talked about "the young-and-temporarily-bi-identified and the gay-and-kidding-themselves-about-being-bi" folks getting into these studies and messing up the results. In a relatively recent study [Rosenthal, A.M., et al., Sexual arousal patterns of bisexual men revisited.
Biol. Psychol. (2011), doi:10.1016/j.biopsycho.2011.06.015] they controlled for these folks and look what they found:
On average, the bisexual men in our sample had distinctly bisexual patterns of both genital and subjective arousal.... It appears that some men may identify as bisexual because they are sexually aroused by both sexes, even if they experience considerably more arousal to one sex than the other. Alternatively, men with bisexual arousal patterns may experience temporal fluctuations in their attractions and arousal to men and to women. Thus, a bisexual man may be more aroused by male stimuli at one time point but by female stimuli at another time point. Further, his arousal to his less arousing sex may vary in magnitude depending on fluctuations in his attractions to that sex at any given time.
The current study establishes that some bisexual men have bisexual arousal patterns. Accepting the centrality of sexual arousal patterns in understanding male sexual orientation (Bailey, 2009), this suggests that indeed, some men have a bisexual orientation.
So, wearytraveler and my other bisexual brothers you have *always* had a place here but now (look science!) you can send the uninformed here to read the research and not concern yourself.