What is the connection between gay and straight, they are variations of for no reason, it is the key. Creatures without fault, which means a species or a group doesn't just variation, they need a form. I am not a student, what your definition is, I'm not very clear. If homosexuality is the result of a mutation out for no reason, then, bisexual men and women may not exist. However, the evidence suggests that homosexuality is not the result of variation for no apparent reason.
Bisexual people do exist, if we look closely straight, you will find, some extreme reject homosexuality. From psychological aspect, there is no fault between gay and straight, is continuous. Now many people admit that this is the psychological deviation, if from the perspective of the pathology from heterosexual psychological deviation to gay, middle inevitably exists in the form of excessive bisexual.
From the mode of thinking, social, sexual interest, aspects of the sexual impulse, require the existence of bisexual people, can make these characteristics completely continuous without fault. The value of love and sexual proportion can only exist in bisexual people's continuous. In my opinion, if there is no existence of bisexual, in many point of view, the emergence of the gay is really only one or a few special gene mutation causes can be explained. So, it is basically impossible.
Due to social reasons, the number of bisexual was found to be not enough, not as evidence of continuous without fault, therefore, this article also lack of proper evidence, only reference for people who are interested in it.