"Some gay friends find members of the opposite sex unattractive."
This phrase alone suggests the initial post is utter bollocks!
And lets notmention the male bias here............
"However I'd guess for the majority of people their sexual preferences remain constant throughout their life." Well, historically that certainly wasn't the case for many lesbians, in that many women only came out or realised that lesbianism was an option after many years of marriage. Seems to be more the case for gay men, but then that's probably because as they are men, they always got more choices anyway.
As regards those worrying about brain sex/hormones/chemistry, my main question would be why do you find this so interesting? Given that historically, heterosexuality was considered the default, hence any change from this was something that needed explaining. I would prefer,. humans are complicated creatures, with all sorts of likes and dislikes and propensities some of us prefer our own sex in various ways, some don't, some of us can do complicated maths, and some claim they can't, some are good at arts subjects, some prefer technical stuff. If you start dividing it up, I have a homosexual brain, I have a maths brain, I have a crap at foreign languages brain -- I just end up at I have my brain with the mix of propensities, skills, experiences and personality that makes up me.
If it was simple a gay gene, we'd have found it by now. Unlike simple things (like eye/hair colour), sexual orientation isn't that simple, and frankly a mixture of genetic propensity, and everything else that ever happens to you from that point onwards!
Then (of course) we have the example of bonbos (pygmy chimps), who have homosexual and heterosexual couplings of many and various sorts.
I can understand why many people go for the 'I was born this way' line (especially when it comes to gay rights), but frankly I'd prefer 'even if I wasn't, what's wrong with being homosexual?'. Myself, I don't know how, I was just lucky! 