While this report about scientists finding a single gene that controls the sexual orientation of fruit flies is quite interesting, I think they perhaps overstate their case a bit. People are far more complex than flies, and if it really was this simple then it would at least seem to make bisexuality (which, according to Kinsey and later researchers, is far more common) impossible.

Then again, there's the less-oft discussed counter-argument... if sexual orientation wasn't at least largely genetic, would men and women really be willing to put up with each other?