Good point suffragette. I was thinking about that, and I do have a hypothesis.
Their environments were almost identical, but identical twins do develop different personalities very early in life. Just because they grow up in the same environment doesn�t mean that they develop in an identical manner. I think that the psychological development of a person may depend on specific experiences and how the person responds to those experiences. So it appears that extremely small differences in environment can lead to large differences in psychological development. This seems reasonable to me. If you hold a pen vertically, resting on its point and then release it, which way will the pen fall? Very small differences in the initial conditions can lead to very different results.
I do want to point out that if sexual orientation is not genetic, that does not necessarily mean that there has to be a conscious decision to be gay. Were you conscious of how your personality was developing when you very young? So I come to the conclusion that that homosexuals don�t make a conscious decision to be gay, but I am not convinced that it is completely genetic either.
Again, I am not a psychologist or a biologist. So I may not know what I am talking about, and I apologize if I have used incorrect terminology. I am interested in doing more research on the subject. It would be nice to hear the view of an ABer who is knowledgeable about the current research on this subject.