It is shaped by both in my opinion nature and nurture.
There is no gay or straight gene. But recently, scientists have found that on sex chromosome X, there may be some genetic influence of sexual orientation. I'm gay and at no time did i make a decision to be straight or gay, I'm sure none of you guys did. If you did, maybe your just asexual. So many tests, especially those of the brain, have repeatedly proven sexual orientation to not be a choice. What causes someone's sexual orientation is likely something not environmental. I say that because people say people become gay if they are molested by a pedophile as a young child. This is absurd. Mostly because I personally know no gay men to whom this has happened to, but i do know straight men whom have had this happened to them. The theories that people propose suggesting environment influences sexual orientation is quite inconsistent. Now i do believe that environment can influence fetishes. If you study the Polynesia cultures that wear no clothes, the men and women have no fetishes and they aren't constantly staring at each other's genitalia. In fact their societies are LESS decadent due to wearing no clothes, because sex is not a taboo subject and filled with all kinds of crap.
No evidence. The human DNA genome was decoded in 2001.
But research suggests that it may have genetic factors involved.
Homosexuality is the desire for someone of the same gender as you. It is scientifically impossible for 2 men or 2 women to have a child together. If people with a "gay gene" were born gay they would not have sex (meaning that they couldn't have kids). Thus, if there was a gene for gayness, it would not be passed on. The gene, if it ever existed, would have died out after the first generation. If gayness is a recessive trait, then its increase in the general population would be illogical (real recessive traits like blue eyes have become less common in America over the years). Furthermore if you were born either gay or normal, how could bisexuals be explained? The only way would be through incomplete or codominance. If that was the case, then, in order to be straight, one would have to be homozygous for heterosexuality. As a result, it would be impossible for 2 straight parents to have a gay child. Thus, it is illogical to conclude that homosexuality is genetic.