The landmark studies were done by Simon LeVay at the Salk Institute and published in Science in the early 1990's.
Here is what LeVay said of his own work in a March 1994 interview with Discover magazine:
"[His 1991 research] made the unassuming LeVay one of the most misunderstood men in America. "It's important to stress what I didn't find," he points out with the courtly patience of someone who long ago got used to waiting for the rest of the world to catch up. "I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn't show that gay men are 'born that way,' the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain --INAH3 is less likely to be the sole gay nucleus of the brain than part of a chain of nuclei engaged in men and women's sexual behavior. My work is just a hint in that direction--a spur, I hope, to future work."Source: Interview with David Nimmons (March, 1994) "Sex and the Brain", Discover, Vol. 15, No. 3, p. 64-71.
There are tens of thousands of former homosexuals. This fact supports environment being the major factor behind homosexuality. This article summarizes it well:
How Might Homosexuality Develop? Putting the Pieces Together
Never has it been sated in the research that homosexuality is genetic. LeVay's work pioneered work into evidence that there is a developmental disorder. Developmental NOT genetic. That is when something goes wrong between the "blueprint" of the gene and the production of proteins and the structuring of cells.
The subjects in such studies are genetically male and have no genetic abnormalities but something went awry in their early development that affected areas within a neural pathway of a neural system that controls sexual behavior. The hypothalamus, the basal ganglia, the amygdala are not 'gay centers' they are portions of a system that effects emotions and behavior. Alter the system - alter the output of that system.
As to "tens of thousands" of former homosexuals - that is a gross exaggeration. There are some who engage in homosexual behavior as a result of sexual abuse from when they were younger. The homosexual behavior is a symptom of a deeper trauma that produces many deviant behaviors. With proper therapy, some of these individuals can become more well adjusted in their sexual behaviors and relationships. This is one reason why homosexual behavior should remain in the DSM. However, that is a homosexual aberration from abuse - not the 'born gay' developmental disorder.
Cults like NARTH have a falsely inflated success rate. In actuality, the failure rate is about 85% and the recidivism rate of those "cured" is extremely high. This is from not understanding the population that requires treatment and the population that simply has a developmental disorder. It is easier to give sight to someone born blind than to 'cure' a homosexual with the NARTH brainwashing methods.
I'll leave you to trolling other old threads.