It seems to me that same-sex attraction, like other biological characteristics, probably forms a bell-curve. The 2 1/2 to 5 % of the population at one end of the curve are obligate heterosexuals, the 2 1/2 to 5 % at the other end obligate homosexuals. The remaining 90-95% are in the middle. However, behavior and conditioning are strongly skewed toward heterosexuality by other factors, including cultural pressures, desire to procreate, etc., et al.
There are other causes of homosexuality as well.
Negative conditioning, for example: A victim of heterosexual childhood abuse, for one example, may find it impossible to relate sexually and emotionally to members of the opposite sex in a healthy way, and may be driven to homosexuality.
Isolation from normal heterosexual access is another--prisons, for example.
Evidently, cultural structuring and conditioning can be a cause. We see this in the reports from Afghanistan and other Islamic countries, and this probably also played a role in ancient Greece.
I believe homosexuals when they say that they have always been this way and cannot change.
This is by no means an endorsement of homosexuality or any homosexual agenda! I particularly do not advocate the public flaunting of homosexuality, and I think that whether or not homosexuals should serve in the military is a decision that should be made by the Joint Chiefs of Staff, according to what is best for the defense of the country, in their judgment, and that their decision should be final.
That may be true in some cases, but Yahoo ran an article last year reporting a study that showed a 15-fold increase in homosexual sex among women over the last decade. This is clearly a culturally influenced change in behavior, caused primarily by the explosion of pornography, in all likelyhood (more women are obviously going "bi" - BOING!!!). Now, there are homosexuals who seem to be that way by nature, but alot of it is cultural.