from "Born that way" theory
Speaking of xenobiotics---environmental pollutants which have a hormone-like effect on the body---Dr. Lee said that when a pregnant woman is exposed to such chemicals, they may blur sex differences in her unborn child. The resulting gender distortions, he theorized, could account for some instances of homosexuality when the child grows to adulthood.
Dr. Lee noted an earlier, similar finding: mothers who took the synthetic hormone DES during pregnancy were more likely to have daughters who developed vaginal and cervical cancer, and a higher-than-normal proportion also became bisexuals or lesbians.
Theorizing that environmental pollutants could have the same effect, he concluded, "If xenobiotics can blur the distinctions between the sexes in seagulls and alligators at nanogram levels, how far-fetched is it to speculate that the same pollutants may be affecting humans in the same fashion?"