Well, in Fruit Flies at least, it seems that some genetic triggers have been isolated.
Featherstone found the gene interesting initially because it has the unusual ability to transport the neurotransmitter glutamate out of glial cells -- cells that support and nourish nerve cells but do not fire like neurons do. Previous work from his laboratory showed that changing the amount of glutamate outside cells can change the strength of nerve cell junctions, or synapses, which play a key role in human and animal behavior.
But the GB gene became even more interesting when post-doctoral researcher Yael Grosjean noticed that all the GB mutant male flies were courting other males.
"It was very dramatic," said Featherstone. "The GB mutant males treated other males exactly the same way normal male flies would treat a female. They even attempted copulation."
This is a very interesting experimental result indeed. Skeptically, one can trivially claim that this has little or no bearing on humanity, after all we are much "more complex" creatures than a mere fruit fly. However, to do so would ignore how much fruit fly based research has contributed to our understanding of genetics, and how that work has contributed to greater knowledge and understanding of the human genome.
Far from being conclusive in the seemingly endless debates over "nature versus behaviour", this little tidbit of research suggests strongly that sexual orientation and sexual behaviours have at least some of their basis lurking in genetics and and assortment of biochemical responses to things like pheromones.
Most interesting indeed.