Reply #47: This might be what geneticists call a "male-killer" gene.

[Odin2005]

Male-killer and Female-killer genes are selfish genes that skew the ratios of fertile adults in a population (before things like in-vitro fertilization was around gay people would most likely not have kids because of no attraction to the opposite sex). Bisexuality my be the result of "countermesure" genes evolved to silence the selfish genes; a person with the selfish gene who also inheirits enough of the countermesure genes would still be straight, pa person with a lack of countermesure genes would be gay, and people with some but not enough countermesure genes would be bi.