Well … mental illness isn't chosen either. At least, saying "Stop being schizophrenic! Just choose to be mentally healthy!" has proven ineffective as a therapeutic method.
Seriously, in many cases, I have to suspect that it's projection.
In puberty we all start to have sexual feelings. And clearly human sexuality is far more complex and nuanced than previously believed. I don't know if it's nature, nurture, or the consequences of little choices made earlier in childhood – probably a combination of all three – but some people find themselves attracted solely to the opposite sex, some are attracted solely to the same sex, some are truly bisexual, some lean one way or the other but have some feelings the other way too.
I pity those who find that they are attracted to farm animals, or corpses, or worst of all children – but I digress.
Now in my case I was (and am) attracted exclusively to women. I saw athletic young men in the showers every summer at camp, and they never did a thing for me; but Dad's stash of Playboys – Great Balls Of Fire. There was zero choice involved and I never had to decide to suppress some feelings and dwell on others.
So when a gay man tells me that he didn't choose to be gay, that he fought it, denied it, would have given anything to be straight – I have no problem believing him. I didn't choose my orientation, either.
But when I hear someone insisting that orientation is a choice, I have to suspect that, for him, it was. As a teen he was attracted to girls and at least a little bit to boys as well – but he denied that part, suppressed it, willed it to go away, chose to be heterosexual. So when he sees openly gay people his thought is, "I chose to be straight, why don't you?"
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"We are fast approaching the stage of the ultimate inversion: the stage where the government is free to do anything it pleases, while the citizens may act only by permission – which is the stage of the darkest periods of human history, the stage of rule by brute force." – Ayn Rand