Well, like so much else, the gender related attractions between people has more to it than the binary Western culture insists on.
There is both romantic attraction and sexual attraction, and they might not be lined up quite the same! Persons may experience romantic attraction towards someone with whom they do not feel sexual attraction, and vice versa. A person may be asexual, no sexual attraction towards anyone, yet still experience romantic attraction.
But wait, there's more! Before coming out, many of us were trying to fit our cultural model, expressing and admitting only strict binary attraction. In coming out, our introspection and self-discovery may reveal a broader range of attraction than we once admitted to.
A fair fraction of folks going into transition experience a 'change' in orientation, as they ultimately uncover new aspects of an underlying orientation broader than the traditional binary.
I've experienced most of this personally within myself and in interactions with others. It's definitely eye-opening, and I find that if I am honest with myself, I wouldn't mind a pansexual romantic attraction, experiencing romantic attraction with other human beings, without regard to cultural labels. I'm not far enough along to know for sure where I may find myself in a few years.