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Sexual orientation is probably the result of brain structure, which is probably the result of conditions in the prenatal environment (e.g., exposure to various hormones in the womb and other things like that). I doubt there's a specific gene that causes homosexuality. I think it's probably more accurate to say that naturally occurring prenatal conditions (in all women, in all cultures) result in heterosexual offspring about 95% of the time and result in homosexual offspring about 5% of the time, give or take a few percent. Either way, the result is a natural, regular occurrence, no matter which section of the gene pool you're talking about, or even which species you're talking about.
In my opinion, the term "homosexual" needlessly complicates the issue of sexual orientation. The issue isn't whether a person is attracted to people of the same gender; the issue is which gender a person is attracted to. Gender and sexual orientation are two separate things; they are not directly related. People who have one type of brain structure are attracted to men, regardless of their own gender, and people who have another type of brain structure are attracted to women, regardless of their own gender. I think it would be nice if we could get away from the emphasis on "same sex" and "opposite sex" and just keep it simple by focusing on which gender people are attracted to. Perhaps that might help to change the terms of the "debate".
I suppose we need special terms to identify men who are attracted to men, and women who are attracted to women. (In the social world, sometimes you need to know where other people stand.) The term "lesbian" is unambiguous, so perhaps it fits the bill (as long as it is used with respect), but unfortunately, the term "gay" lacks the necessary precision to apply specifically and exclusively to men who are attracted to men. For the same reason, the term "homosexual" isn't any better. Does anyone have a better suggestion? (I'm seeking an answer that is dignified and respectful.)