The OP doesn't seem to hold a hypocritical view, neither is he imposing his view on other people. He isn't saying that all people are not born gay, just himself. People often view nature and nurture as two mutually exclusive categories, which seems far too reductionist to me. People are born with certain dispositions, which the environment then acts upon. Even genetics are not set in stone at conception - genes can be turned on or off by the presence or absence of certain genetic factors. Take the example of the development of language, our genetic make up allows the capacity for its development, but certain environmental stimuli must be present during the critical period for that capacity to become actuality. Another common misconception is that things that are caused by nurture are also necessarily down to choice.
I think, like most things, sexuality is determined by an interaction between biological factors (genetics, and biochemical conditions in the womb and after birth) and environmental factors (such as the niche you must take in your family). I highly doubt that sexuality is ever, at least directly, chosen by the individual. That said, I think sexuality can be determined from a very young age. I myself am a lesbian, and for as long as I can remember I have been very butch (not that all homosexual people are gender nonconformists by any means, but a lot (75%) of people who were gender nonconformists in childhood grow up to be gay).