Is Sexual Orientiation genetic or is it a choice?

This simple question does not have a simple answer.
Genetics is the domain of medical science. What has science found regarding sexual orientation? First, science has found that sexuality and sexual orientation are some of the most complex aspects of people and the answers to simple questions like "What causes sexual orientation?” are not simple.
Some researchers have studied twins. The study by Bailey and Pillard, published in 1991, reported that when one twin is homosexual, the number of times the other twin is also homosexual occurs more often than the general rates of homosexuality in society. The media reported this as proof that sexual orientation is genetic. But Bailey and Pillard themselves clearly stated that this study did not prove that homosexuality was solely caused by something genetic. Interestingly, this study also looked at the rest of the family and found that the rate of homosexuality among the rest of the family, including adopted brothers was 200-300% more frequent than the general rate in society. And so the data indicates that something is causing homosexuality to occur more frequently in the families that were studied but that it can not be genetics because the increase also occurs in adopted brothers who do not share any genes at all with the rest of the family. From six studies between 2000-2011 researchers have concluded that if one identical twin has same-sex attractions, the chances that the co-twin has it too are only about 11% for men and 14% for women. This indicates that factors the twins have in common, such as genes and upbringing are mostly not responsible.
Others have studied hormones and brain structures looking for other possible causes that are biological but not genetic. While they have found there may be biological differences between heterosexuals and homosexuals, they can not determine if these possible differences cause homosexuality or they are the result of homosexual activity. Research on the effects of exposure to testosterone in the womb shows that its influence on the formation of opposite-sex attractions is weak to modest - only 16-27%. The influence on the formation of same-sex attractions homosexuality and transgender orientation should be even less. 
Still others have studied sociological and psychological causes for sexual orientation. They found substantial evidence that sexual orientation is developed and that it is not fixed but changeable. However they have not found that sociological or psychological causes are determining factors.  
When all of the work being done is put together, the question about what causes sexual orientation seems best answered as follows.
First, science does not have a firm answer that everyone in the field of science, let alone everyone else, can agree on. The origins of sexual orientation are still not clearly understood by scientists.
Second, most researchers agree that the causes of same-sex attractions are multi-causal and complex; many factors contribute to the development of same-sex attraction. Most researchers, including Dr. Dean Hamer, the "gay gene” researcher who is himself a gay man, agree that same-sex attractions are due to a combination of social, biological, and psychological factors. Dr. Hamer has said, "Genes are hardware…the data of life’s experiences are processed through the sexual software into the circuits of identity. I suspect the sexual software is a mixture of both genes and environment, in much the same way the software of a computer is a mixture of what’s installed at the factory and what’s added by the user [P. Copeland and D. Hammer, The Science of Desire, New York: Simon and Schuster, 1994]”
 It is also important to note that the combination of factors is different for each person.
Now the second part of the question - is sexual orientation a choice? No it is not a choice. No one is able to decide on a specific day that from that day onward they will experience same-sex attractions or opposite-sex attractions. People do not consciously decide their sexual orientation. The mean age of first same-sex attraction is 10 and two thirds of the ages of first attraction are in the range 6-14 years. There can be little informed, responsible choice involved if first attraction is about age 10. At that age no-one chooses lifetime sexual orientation or lifestyle in any usual sense.
So, returning to the question - is sexual orientation genetic or is it a choice? The answer is neither. The causes of same-sex attractions are multi-causal and complex, and we should not adopt a simplistic either/or approach when looking for what causes sexual orientation.

"Perhaps one of the biggest concerns for the person on the street is whether we are stuck with our genetic inheritance, or whether we can overcome our genes. Dean Hamer stated, ‘One of the biggest myths is that something genetic is therefore fixed. This simply isn't true. It’s what we do with our genes that matters. Someone who relishes novel experiences might use this trait for good or for bad — to become a great explorer or a violent criminal. All these genes do is to give us a disposition one way or another. Whether we act on that —or don’t — is very much a matter of our free will.’ [The Power of Our Genes: An Interview with Dean Hamer, Science & Spirit, December 1998].

For more information about the roots and causes of homosexuality, please go to the Articles section.