The evidence is growing that homosexuality is a birth defect due to hormonal conditions of the mother while the fetus is developing. The ratio of finger lengths of gays is different that straight. Gay men are more likely to have hair on the back of their head curl counterclockwise than straight men. Both of which are biological markers affected by hormones. Women who smoke or take estrogen are more likely to have gay children. Women who have multiple children are more likely to have a gay child due to their immune system reaction to male hormones. The brain structure of gay men has been shown to be similar to that of women compared to straight men. The increase of synthetic hormones in our bodies due to plastics, hormone treated animals has been shown to have an impact on fetus' in pregnant women.
Granted, it isn't a crippling birth defect being more along the lines of male pattern baldness than say Downs' Syndrome.