Featherstone himself has a pretty good page explaining this at http://tigger.uic.edu/~def/ It's in lay language and well-written. The key paragraph is this: "Drosophila, like us, rely on glutamatergic synapses [note: a particular kind of amino acid] for brain function and control of complex behavior. In fact, essentially all of the same molecules and mechanisms used to build and operate human brains are also used in flies. For example: Drosophila learning relies on the same particular type of glutamate receptor relied upon by humans. Also, 87% of the human genes associated with mental retardation have equivalents in flies (many of these human genes were even named after the fly gene!) The human-fly similarity at the gene level holds not just for the brain; 75% of genetically-linked human disease genes have matches in flies." What this shows is that if you make genetic changes in certain receptors, the sexual orientation of a fruit fly can be changed. This STRONGLY implies that there are similar types of receptors in the human brain that control sexual orientation. It's of course probably not nearly as simple as it is in a fruit fly, and of course affectational preferences and perhaps even gender identity creates a far more complex set of behaviours in humans, but it still gives us an idea of how and where some aspects of sexuality is hard-wired in humans.