In 1991 an article made by Simon LeVay, professor in neuroanatomy by the MIT in Boston, implied that partner preference is genetic. There are a core of cells in the hypothalamus (INAH-3
http://en.wikipedia.org/wiki/INAH_3 ) is said to be smaller if your prefer a male (sexual) partner. In context, you could say that the INAH-3 is smaller among homosexual men, than the INAH-3 among heterosexual men. This also goes for homosexual women; A homosexual woman has a bigger INAH-3 than a heterosexual woman.
The INAH-3 is developed already at the stage of an infant (based on the level of testosterone produced) and therefore you are born with your sexuality, and homosexuality should therefore be observed as a normal variant to heterosexuality. This testosterone production in such an early stage of your life is also said to determine whether you end up with a female or male structure of your brain. So to imply that there's no such thing as "bisexuality" is wrong. As a matter of fact, neurologists with a psychology orientation imply that 25% of all the worlds population probably is bisexual and that roughly 10% of every male that is born is homosexual.
And here is where the social environment starts to play its role. Due to the norms of today's society homosexuality is frowned upon and thus oppressed by people having an "alternative sexuality". A man can be perfectly homo/bisexual, but learnt in an early stage of his life that homosexuality is wrong and "unnatural" and thus he lived his entire life completely believing that he's heterosexual. He even ended up married to a woman and had kids.