It is not a choice but there is no evidence that sexual orientation is purely determined at birth (i.e., purely genetic). We certainly have not found all the genetic markers associated with same sex attraction so there is more to learn but currently [as of two years ago when I did a complete literature review for a postdoctoral project] the best research shows that homosexuality is 30-40% inherited (meaning genes explain up to 40% of variance in expression of homosexuality). This is only true for males. For females, there is weaker evidence homosexuality is genetic - up to about 20% of orientation can be explained by genes [range from studies is 0-40% genetic; most group in the 10-20% range]. There's still a lot of research to do but no research will ever show sexual orientation is fully genetic; in fact, environmental factors play a larger role (again, based on the best research to date - this might change in the future with more studies).
Here's a quote from a pretty good twin study in Sweden published in 2010: "Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior." (Långström, Niklas, et al. "Genetic and environmental effects on same-sex sexual behavior: A population study of twins in Sweden." Archives of sexual behavior 39.1 (2010): 75-80.)
What this does not mean is that sexual orientation is necessarily a "choice"; it does mean, however, that for practically all people who are homosexual it is not correct to say "I was born this way". "I developed this way" is more accurate. Genes and environment (nature and nurture) both play a role. Choice even plays a role. I'm not calling sexual orientation a lifestyle choice, I'm pointing out that choice always plays a role in the interactions between genes and environment. That choice is (or those choices are) not always being made consciously and is not always being made by the individual; it can be made by others.
This is a very complicated topic with many unknowns. The research, frankly, is too subtle for most people to appreciate. That's generally true for all research - how do you boil down concepts that took the scientist years to understand and/or develop into a blurb that the general population will understand? This means for homosexuality, saying "I was born this way" is easier and helps lead towards better treatment of homosexual individuals.
/I did not use the term LGBT because bisexuality and transgender are not the same when talking about genes and environment. Even less is known about genetic links of bisexuality and transgenderism.
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