Myself I can offer anecdotal comments only. That being said, our family has much experience with this question. Most recently, a niece of mine—a beautiful twenty-one year-old half-Egyptian girl, received an ultimatum from her longtime boyfriend: break up with me and I'll turn gay. Well, break up with him she did. A few weeks later he was dating a guy.
Clearly, the 'guy' was bisexual, and did not, in point of fact' turn gay'--he just withheld that fact about himself to her. As member of the male species, I have it on good authority that males are very capable of being untruthful or outright lying about sex. Heck, a male will lie to God, and Congress, let alone to a girlfriend, on the subject of sex. I can't speak for females, however.
In my family an uncle, an aunt and a cousin are gay—have been for decades. The uncle used to date women up into his thirties, he's sixty-one now, but came out around age thirty-two and stayed in a relationship a guy for twenty years. Since then he's been seeing guy after guy, all of them much younger but over age twenty-one. Stories of his childhood vary depending on who one asks, but the consensus seems to be he was genuinely interested in women at some point in his life. Who knows what happened to change his preference. Either way, he's always been a great uncle although he went totally flaming in his later years.
It happens in some gay men, who start out predominantly heterosexual, and their homosexual desires are fleeting at first, but grow over time and take full bloom by the time they are mature adults, some merely remain bisexual.
The aunt married my biological uncle, had two children with him, stayed with him for twelve years and then ran out on him for another woman all of sudden, leaving my two young cousins without a mother for about three years before coming out to the rest of the family. Both cousins suffered greatly in her absence and her son eventually claimed to be gay as well and has been with men since about age nineteen. That aunt's brother is also gay, as are a few other men in her family.
Drawing simply from personal experience, I've always concluded people choose their sexual preference due to some great upheaval or realization or influence in their lives. I do not believe people are born gay. However, as mentioned, that's what I believe based on personal interactions with gay people, most of whom I am related to, some of whom I have known since (my) earliest memories.
They don't actually choose, it's inherent. It manifests in varied ways, just as plants grow and achieve fruition in varied ways, but the plants do not 'choose' how they manifest, it's inherent-it's dna/gene driven or, perhaps, it's inherent in the 'soul' (if you believe in that sort of thing).
And there are variations of homosexuality. Some men are bisexual. Some men's bisexuality is not concomitant/contemporaneous, i.e, can have sex with both males and females at the same time, where they just 'flip' one day they are gay, another they are straight, but never both at the same time. Some men can be attracted to other males, but only extremely effeminate males, yet others, the opposite. Some gay men prefer transfemales, but only those with a penis. Some gay men are bisexual, but their attraction to the same sex is limited to pure sex, lust, their 'gayness' is more of a fetish, as it were, but when it comes to romance, handholding, kissing, affection, wanting companionship, they are not 'bi-romantic' and reserve that type of relating to the opposite sex.
Yet some gay men are totally homosexual, and want what a woman would typically want, the arms of a man, his love and affection and the desire to create a family. I knew bisexual guy who told me he totally abstained from sex, because he had a strong desire to copulate inside a man, an effeminate man, but at the moment of copulation, his mind would flip to being straight, and the sensation of copulating in a male while he suddenly found himself heterosexual again was a horrific experience, such that he became celibate. He found that he was attracted to women in an unaroused state, but attracted to men when he was aroused, so, that when he was aroused, ie was always gay, and his mind would switch back at the moment of ejaculation. I felt so sorry for the guy. Nature plays mean tricks on some folks. Sex is a weird thing, and there are probably more variants I've yet to learn about, though no doubt there are books and videos describing other variations I've never heard of.
I lived in West Hollywood for 20 years, having been involved in the arts (music and photography), and have had many gay friends, and have learned these things from them.