As stated in this excellent LA Times op/ed,
Moreover, the empirical evidence for the role of genetics in human sexual orientation has been quietly but steadily mounting over the last 15 years. Studies of twins -- the mainstay of quantitative human genetics -- have been conducted on large populations in three countries. The results unambiguously demonstrate that heritability plays a major role in sexual orientation and far outweighs shared environmental factors such as education or parenting.
I'm focusing here on genetics, which is my field. For other general audience discussions of the science of orientation, there's a good article in NY magazine: The science of gaydar.
As I post my Genetics series, I will add the posts to a static page (see that tab for "Genetics Page" at the top of the blog) so that the whole series will be accessible in order.
There will be no examinations, and you don't have to pay tuition. What a bargain!