There is evidence that both genetic and environmental factors play a role in homosexuality. Someone who has been exposed to a significant amount of the mothers antigens would develop a higher degree of the characteristics associated with the opposite sex and have a higher chance to express homosexual or bisexual attractions in proportion to the general population. However, this is but a factor that merely increases the likelihood of expression of homosexuality or bisexuality, most who fit the above criteria are not going to express the traits of homosexuality or bisexuality. A second factor that increases the likelihood of expressing these types of sexual deviancies is being the victim of sexual abuse perpetrated by a member of the same sex as a child, which increases the chances of homosexuality or bisexuality in the victims proportionate to the general population. Yet again this is just a factor that raises the likelihood, and most child sexual abuse victims of a same sex partner will be heterosexual. A third factor is exposure to agents that stimulate feminization in males or masculinization in females, aka exposure to chemicals and other elements that cause the development of sexual traits associated with the opposite sex, such as breasts on chromosomal males and facial hair on chromosomal females. The younger the exposure and the greater the degree, the higher the likelihood of developing homosexual or bisexual behavioral traits. Most will not be homosexual or bisexual, however. Fourth factor is social environment and the associated cultural pressures, those who are more exposed to pro-homosexual or anti-heterosexual memetics are more likely to express homosexuality or bisexuality than those exposed to the opposite or to none at all. The influence of society on people's sexuality is noted to be quite significant. And in this case, once again, most will be heterosexual. So in sum, it is not strictly internal factors or external factors that make a gay or lesbian, instead it is likely an interplay of these that produce such traits in people, those who meet multiple such criteria are much more likely to be sexually deviant than those who only meet one, and the more criteria are met, the greater the chance of becoming this way. Not that any of I matters, the whole debate is a giant red herring, in no other context is the cause ever considered important for considering some behavior as mental illness. Schizophrenic are born that way, as are autistic, and most recognized mental abnormalities in the DSM, never once is the inference of these conditions considered a valid argument for their removal from being classified as mental disorders. The reason we classify things as mentally illnesses is because they produce issues for the one afflicted with them and for those around them in society. Nowhere dies the idea of a biological origin ever factor into this classification. And on this basis, homosexuality and bisexuality both fit the defining criteria for mental disorders, as they negatively impact both the affected and those who encounter them. "Born this way" is a devious argument with a false premise that most who debate the topic will passive accept like a chump. Stop pretending ding that it matters at all whether this condition is congenital, and focus on the fact that the negative traits associated with the condition make it not only fit for classification as a disorder, but also unfit for declassification.