There is new research that shows that bisexuality is real, and not merely a phase -- at least in women. Those of us who have more than a passing familiarity with our closest relatives -- the chimpanzees and bonobos -- already knew this. Bonobos especially are as a species bisexual in nature. Chimpanzees are less so, but human behavior tends to fall between that of chimpanzees and bonobos, though more on the side of chimpanzees overall (we're more aggressive, like chimpanzees). Bonobos use sex for everything -- to trade for something they want, as a replacement for aggression, etc.

The research in question, though, only goes so far as to suggest that bisexuality is an orientation, though they do note that there is some fluidity in female sexual identification. I am convinced that this fluidity is more pervasive than we think, and that various cultural norms have pushed humans into expressing our sexuality in more either-or fashion than we otherwise would. Certainly heterosexuality dominates as an overall sexual tendency, but I think it's a sort of off-center bell curve, with various levels of bisexuality along the curve. Homophobic reactions are typical in those who are in the mean of the bell curve, as the feelings are rare, but there, and can freak people out -- especially in anti-gay cultures. Very strongly heterosexual people would likely not be homophobic, as they are in no way "threatened" by such feelings, and are neither turned on nor disgusted by homosexuality (the disgust may, in fact, be the flip side of being turned on).