Yes, dittany, they have found a tendency for a certain genetic pattern in homosexual males. And no, that doesn't mean you can do a 'test' for it. In the same way that having a gene that makes you susceptible to cancer doesn't mean you'll get it. Environmental factors play a part.
I agree with you, though MillyR, that giving homosexuality a biological cause isn't ultimately helpful except to enlighten people like my mother who are still convinced it's just attention-seeking behaviour. I would say, also, that your point about how some people are most certain about their sexuality than others supports my position that sexuality, as with gender, is a spectrum, not an absolute.
As for your previous post, MillyR, I brought more 'fixed' human traits such as height into the equation just to demonstrate that very few of us are ideal in terms of human fitness. It had nothing to do with the sex or gender argument. I will point out, though, since you fixed on height, that this too is a plastic trait. This generation of Oriental people are much taller than previous generations due to a more Western diet.
However, having given this more thought. I do maintain that gender and/or next are not nearly as neatly fixed as some posters would like, and there are in fact a range of points on a spectrum between fertile heterosexual male and fertile heterosexual female. We're still all people, not two different species fighting over the same ecological niche. But, it has occurred to me that our current social construct doesn't help males with a more feminine thought-pattern, or vice versa. If society was more accepting of men in make-up and dresses who like to bake cupcakes, and of women with cropped hair and biker jackets, accepting them as completely normal, perhaps these people would be more like to be happier as their assigned gender. Perhaps in society as it stands, they feel it easier to actually have their genitalia mutilated than live as an effeminate man or butch women, such is society's condemnation of them.
But since I'm not a transgender, and have never discussed this with them, I could be talking utter bullshit, so I won't speculate further.
Anyway, I've spent too much head space on this, so I won't post further, though I will continue to read with interest.