Like much in the field, this is something of an open question. You can fill literal libraries with material on this subject and still not get a clear cut, concise answer to this.
However if you honestly believe it is a choice, then I have to ask you when you decided what your sexuality would be. Because chosing your sexuality is an experiance I cannot relate to.
It's likely a mixture of genetics, hormones, and social factors. What's interesting to me is that homosexuality can be found in hundreds/thousands of animal species from high-order mammals to cephalopods. Whether or not being gay is a choice or is genetic or whatever else, it's clearly natural.