Bisexual Species: Unorthodox Sex in the Animal Kingdom - Homosexual behavior is common in nature, and it plays an important role in survival (Scientific American, July 10 2008)

"same-sex couplings are surprisingly widespread in the animal kingdom (...) individual animals generally cannot be classified as gay or straight: an animal that engages in a same-sex flirtation or partnership does not necessarily shun heterosexual encounters. (...) animals may engage in same-sex couplings to diffuse social tensions, to better protect their young or to maintain fecundity when opposite-sex partners are unavailable—or simply because it is fun. These observations suggest to some that bisexuality is a natural state among animals"

“[In humans] the categories of gay and straight are socially constructed"
-> so humans are all - to various degrees - bisexual? Could very well be true.