Sexual orientation is a polarizing and controversial topic of discussion in American society. Sadly, there is not much concordance between people about the origins of one's sexuality: many still believe that homosexuality is a choice, whereas modern science and Lady Gaga give evidence of people being born this way. It has entered the sphere of social politics, where perceptions of homosexuality have been clouded by heteronormative personal convictions and outdated religious dogma. It is my personal opinion that religion impedes the progress of humanity, so I will focus only on the biology of sexual orientation.
Many hypotheses exist for how sexual orientation is gained. In our textbook, it gives birth order, exposure to hormones/pheromones in the womb, early upbringing, and temperament as possible factors in determining it.
One hypothesis I found particularly interesting was that homosexuality was caused by a pathogen. As a microbiology major, I found the 'gay germ hypothesis' interesting. Gregory Cochran and Paul Ewald argued that evolution would strongly select against homosexuals, who have lower fitness (i.e. reduced chance of reproducing and passing down genes). This would make sense. In my biology class, however, we learned of kin selection- in this case, fitness of the homosexual's family increases because it devotes time to caring for its relatives' young, allowing their similar genes greater chance to be passed down.
Furthermore, they argued that higher prevalence of homosexuality in dense, urban areas suggested an infectious agent at work. However, you can't infer causation from that correlation- there might be another reason why there are more homosexuals in big cities such as bigger cities being more accepting and/or increased proportion because of a higher population.
The gay germ theory has largely been rejected by the scientific community because of lack of peer review and replicability. The American Philosophical Association said "there is ultimately very little to be said in favour of these contentions" that liken homosexuality to a disease that needs to be treated.
An Evolutionary Look at Homosexuality by Gregory Cochran: http://home.planet.nl/~gkorthof/backup/An_Evolutionary_Look_at_Human_Homosexuality.htm
Crain, C. "Did a Germ Make You Gay?" in Out Magazine, August 1999.