Certain disorders--down syndrome, kleinfelters syndrome--are the result of genetic anomalies. You can change the environment around and the person would still have down syndrome. This is easily observable. However, to state that someone is born gay is almost impossible to prove, i.e., it is not a choice for them. If homosexuality is a mixture of genetics and environment, then perhaps there is a certain amount of choice involved. If you change the environmental parameters, you alter the response of the individual. Perhaps altering the environment can be a deciding factor on whether or not one becomes homosexuality.
Most people that study human behavior agree that behavior is a complex and dynamic process of biology and environment. Back in the 50's and 60's you had two schools of thought. Those that said human behavior was ALL biological, and those that said behavior was ALL environment. It is not like that any more. There is a definite interaction between environment and biology. With that in mind, its nearly impossible to assert that homosexuality is not a choice.
