They can also like both genders equally. The point is, you don't choose who you like, wouldn't you agree?
Perhaps, but how do you know bisexuals do not want to..."explore". It seems that while there may be cases of a Bisexual not knowing which gender is preferable, there may also be as many cases of Bisexuals drawn to the pleasure of having sex from both genders. You cannot claim that one person is Bisexual, simply because they like men and women equally. That, I think , would rarely be the case. Rather, I think it is more likely that Bisexuals engage in intercourse with both sexes for pleasure.
Either way, it hardly matters. If Bisexuals also suffered from a genetic disorder, then wouldn't that mean they'd exhibit a "normal DNA pattering" and a "abnormal patterning". Since neither this nor a Gay Gene has been found, one can logically conclude that heredity plays little to no influence on the likelihood of becoming homosexual.
But I must say that I'm a bit skeptical that it is really the accepted view that animals can go against "fitra".
Can a animal truly comprehend how its decisions will affect it later in life? No, animals, while aware of their place in the world, act in accordance to primal urges. Sex is one of the most fundamental of these urges. If an animal succumbs to this urge, and it will, then it will not think anything of it. If there is a lack of females then it will engage in activities in accordance with homosexuality. Strictly speaking of course, it is not homosexual until the Penis penetrates the Anus, the point Fatihah was making. So while they may engage in sexual activities, the question whether they are homosexual in an Islamic sense was what was being debated prior.