Seem unlikely? The chances of all gay people claiming that they were born that way, and no significant number of them claiming otherwise, indicates that there is either a large, unlikely conspiracy, or that they really don't choose it. If you had to make a bet, you'd have the best odds placing your money on "they don't choose it".
I received an A on my thesis, actually. Here, have a source:
http://serendip.brynmawr.edu/bb/neuro/neuro05/web3/kgiamanco.html
That's the thing: They came out! You see, in some parts of the country, there is an extreme intolerance of homosexuals. A homosexual person can thus be pressured into marriage (yes, it happens). Once society becomes more tolerant, they can then come out.