In keeping with my theories on gender-neutrality (and I separate gender from sex), I've had a long-standing inclination to believe that human beings are naturally bisexual. This is hard for me to swallow myself, since I don't seem to be. But at the same time, perhaps it can be explained in terms of the fact that human beings are omnivores: however, some people are vegans, and others are not. Some people make a value judgment via veganism (if you eat meat, you'r evil and horrible!), and others make a similar judgment via heterosexuality.
Similarly, I'd agree with The David and kljsadfhklj in saying that sexual orientation or preference can be changed, again keeping the analogy with dietary habits. We can certainly become vegans, or we can be raised vegans and introduce into our diet some meat.
This makes sense to me.