In general one example is thalidomide. It produces non-genetic but still congenital birth defects.
As to specifically homosexuality? I have no idea if it is genetic, non-genetic but congenital, post birth mental defect or a choice. I wouldn't be surprised if it is a combination from all of those and more. I was just refuting the author's erroneous assertion that all birth defects must have genetic causes.
Interesting that you would select thalidomide as an example. Most of the afflicted were born in Japan after WW2 and the atomic bombs were used as the primary “excuse” in those cases, or at least that is what I have learned, and that information may be in error I will admit. In any case, such a correlation was determined very quickly and the man made compound was taken out of use for pregnant women.
While there is also a distinct possibility that, chemically and a result of man made compounds, such a cause has not been located to date, and homosexuality has been with humanity from the beginning it seems. That reality makes me believe that perhaps it is simply a learned behavior that is easily acquired or perhaps a common defect of the most complex organ known to man (our brain(.
Interesting speculations, and I still reserve the right to ask any homosexual advocate that if there is a genetic defect or chemical reason for homosexual behavior that could be cured by medicinal therapy or genetic therapy, would they be an advocate for such a “cure”?