Decades of research to discover a "gay gene" have come up empty. In fact, it's uncommon for researchers to think that a gene can determine any behavior. And research that correlates brain anatomy and physiology with homosexual behavior doesn't prove causation. In other words, even if part of the brain in homosexuals is different from that of heterosexuals, that might suggest that their behavior changes their brain, not necessarily the other way around. This is possible due to neuroplasticity the lifelong ability of the brain to change in response to the environment, brain injury, behavior, or simply acquiring knowledge. The blind, for example, use their fingers to read braille and consequently have different brain morphology simply because their behavior differs from sighted people.

What's surprising is that this is acknowledged by pro-gay researchers and organizations. The American Psychological Association (APA), for example, once held the position in 1998 that, "There is considerable recent evidence to suggest that biology, including genetic or inborn hormonal factors, play a significant role in a person's sexuality." However, a decade of scientific research debunked this idea and caused the APA to revise their position in 2009: "Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors" [emphasis mine]. A pro-gay group like the APA would revise their statement only if there was overwhelming evidence that necessitated a position change.

A second problem with the claim that homosexuality is biologically determined is that even if true, it wouldn't prove that the behavior is moral. Consider that researchers have discovered genes they believe contribute to alcoholism, unfaithfulness, and violence. Are we to believe that because there is a genetic contribution to these behaviors (or even if they are genetically determined) that they should be regarded as morally appropriate? Of course not. So, proving homosexual behavior is appropriate by appealing to a genetic determinant is equally spurious.

Indeed, finding a genetic cause to homosexuality worries many gay rights advocates. Why? Because not all genetically induced characteristics are normal or healthy. Abnormalities in an individual's DNA, whether by mutation or the addition/subtraction of a chromosome, lead to genetic disorders such as cystic fibrosis, Down syndrome, or cancer. If a genetic origin to homosexuality is discovered, the next obvious question that arises is whether it is abnormal, a mutation, or represents a disease process.

From: Are Homosexuals born that way?