Greg Cochran first proposed that homosexuality is the result of an unknown pathogen. Here’s JayMan’s write up on it from earlier this year. The idea that pathogens could be the root cause of things like heart disease and insanity has been kicking around for a while, but I know of no serious science on it. Like a lot of speculative science, there’s no money in it until someone finds something concrete. There’s a lot of politics to science because scientists need money to do their work. There’s funding available for genetic research. There’s no money for locating the gay germ.