I don't think it is mostly genetic because straight couples can have homosexual children and probably the opposite also holds true. Besides I can't see the same mutation arising generation after generation, no matter how simple it actually is, that would be some kind of darwinian miracle. Hormonal imbalance would be a good theory if I hadn't met more exceptions to the rule than actual cases, assuming all the thing about how testosterone and estrogen affect growth and metabolism are actually true.
Psychological causes... maybe. I know it would be easy to look at someone who was straight and then became homosexual with time and claim she was homosexual all along, but say, we don't look at someone whose personality changed with time due to traumatic experiences and claim she was actually evil or disturbed since birth do we.
So psychological causes would be a good one, but I don't know. I mean, there are all sorts of people out there, girls who like boys who look like girls and vice-versa... or those who like people with certain features independently of sex... hell it's complicated.
