As humans, we like to have things in order with nice definitions. This is how we evolved, we like routine and processes.
Unfortunately, nature doesn't follow all of the terms and rules humans have built up in order to survive in large groups. Sexuality and a person's "gender" identity are not clearly defined in nature.
It is not just staight, bi or gay. Nor is it just transgender, transexual or "normal". It is a spectrum, people land all over the place and our insistence on finding a definition to apply to everyone may help us make sense of our world, but it doesn't help us understand each other.