Your thesis depends heavily on homosexuality being the result of...

Your thesis depends heavily on homosexuality being the result of a gene inherited in an autosomal dominant fashion.

I don't know about required prevalence of a recessive "gay gene" to explain the numbers of people reporting they are homosexual but I highly doubt if there are solid enough evidences to support even a recessive gene theory.

Given that the gene theory is in my opinion not correct, your example doesn't really benefit from using homosexuality as a "proof".

I think all it shows at best is that homosexuality is probably a chosen ir adopted behaviour rather than something intrinsic to the organism.