Referring to another thread, I was talking about some theories
that suggest homosexuality might cease to exist in the
future.
In the field of Psychology, it is becoming impractical to ask
question like Which is most important, genes or environment? This
is because many researches had already proven that there is no
clear cut to this question.
Human behaviours are the result of complex interactions between
various factors. Only in very extreme cases can one perspective
declared sole responsible for certain behaviours, for example in
the case of serve brain-damaged patients. So for example in this
case the question we should ask is can we understand sexual
orientation based on genetic differences between individuals?
So is there such a thing as a gay gene?
Simon LeVay (1993) in his book The Sexual Brian
suggested that homosexuality might have a genetic origin. He feels
that most of us choose to perceive gays/lesbians as committing
immoral sin or suffering from mental illness. This is not
true.
Studies of twins show that the tendency to either heterosexuality
or homosexuality is higher in identical twins than in fraternal
twins (non-identical), i.e. genetic factor.
Another source of evidence come from the brains of adults at
autopsy. It was found there exist differences in the structure of
certain regions of the brains between gay and non-gay.
On the other hand if there is a gay gene, how does it spread into
the population? From the evolutionists perspective especially
sociobiology, the existence of a gay gene just doesnt make sense.
A gay gene will result in biologically unproductive sex and thus
end up in its own annihilation. Thus homosexuality could not be due
to biological factors.
But for some of them, they suggest that maybe there is the
possibility of a gay gene but eventually through repeated cycles of
being re-created by mutation, it may be eliminated from the
population.
Personally from a humanistic perspective, whether is there any gay
gene is really not that important. As human being, we make choices
in deciding whom we are and what we want to do with our life. As
fellow human beings they have the right to live their life to the
fullest just like every one of us. 
Just my view. 
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References
LeVay, S. (1993) The Sexual Brain, Cambridge, Mass., MIT
Press.
Stevens, R. (1996), Understanding The Self, The Open University,
Walton Hall, Milton Keynes MK7 6AA