There is certainly a degree of uncertainty about the exact mechanisms that determine sexuality.
However, it seems strange to me, if that is the case that one can choose, that people do choose at an extremely early age and that there appear to be significant physiological differences between gay people and straight. It also seems strange that people would choose to be homosexual, particularly when such a choice would inevitably lead to conflict, and possibly even violence (even death) in some cultures, and yet such 'choices' are still made.
I'm sure there are environmental factors at play too, but to my mind, it seems convincing that sexuality is overwhelmingly biologically determined.