(Obviously "fluidity" of sexuality does not preclude "born this way" - it could be that it's written in your genes that your sexuality would/will change throughout your life, similar to many people's libidos and/or hair color.)
Edit: on second thought, it's very possible that it could be random environmental influences (e.g. amount of... I don't know... drums + beech pollen ... the fetus is exposed to in the womb) - that could be pretty much orthogonal with genetics, while still leaving the person with "no choice". It's just that AFAIK most personality traits have a significant genetic component, either actually genetic or epigenetic [1].