Before you go off and study the genes responsible for mental illness in a homosexual population, what would attract you to this line of investigation?
There's no riddle. Some people like crazy buttsecks, or drinking from the furry cup. Some people don't. We is what we is.
What would you do with the answer if you got one? Tell all those poor gays that it's OK, they're just insane in the brain?
You might just as profitably investigate why some colonial weirdos like Vegemite and Superior Imperial Beings prefer Marmite.
And finally: if it's not a choice, why do you advocate discriminating on the basis of it?