Yeah wonderful thoughts, making me think even more about the thoughts I have often pondered. I believe you are born gay; for some that switch is automatically on and they are aware from an early age, others fight or suppress it until that special connection is made with someone of the same sex and that light in the mind is switched on; Ennis even though he would have accepted the fact about his male/male attraction still fought with it until it was too late.
I think it is not a matter of being born gay or straight. The biology, the biochemistry, the genetic basis of a person being gay or bi or whatever, is not totally clear. There are indications that some parts of the brain associated with emotions are more similar being gay men and straight women, and straight men and lesbians, but there is no evidence that this is a predisposition or a cause, only that, by the time of death (because all the evidence was gather post-mortem), the brain is indeed different, indicating some biological component of sexuality. There is no surprise there.
I do agree that there is a genetic component to being gay, and I like your analogy of a switch being turned on. It is possible that the genetic component of being gay is dormant in most people, if not ALL people, and that depending on the person, the switch is more easily or more difficult to switch on. I think all we can do is speculate until more scientific evidence is gathered.
I like the continuous spectrum of latent gayness, and the disparate difficulty in switching on the latent potential. It explains a lot of empirical observation of people's sexuality and behavior. It is a simple but far-reaching theory.
To bring it back to topic, I think Ennis has more latent potential than his brother, who seem to me to be very straight in the short story, a chip off the old block, whereas Ennis seem to me to be more sensitive, at nine, and emotional. Perhaps it was his affinity to the queer old birds Earl and Rich that prompted his old man to murder Earl. Who knows for sure? The traumatic scene, however, I dare say made the switch more exposed, ready for the right person to come along. Jack was the right person, in the right place, at the right time, and did everything right. In my opinion, that is why they are soul mates, because Ennis also gave Jack what Jack needed, faith and trust and loyalty. To be pissed on and ignore most of his life, Jack was thrilled to be the center of attention of this capable rock solid cowboy. To be appreciated, to be needed, to be loved. That was what Jack needed from Ennis. That is what Ennis realized too late at the diner, when Cassie said, love is not about fun. That is when Ennis realized, he too needed Jack. That's when he began the journey of accepting himself as a gay man, even though, I still insist, that in the summer of '63, before he met Jack, Ennis was sexually ambiguous and confused. The switch could have stayed off. But then he would never be as happy as he was by the river, sending up a prayer of thanks to heaven. Or, would he? What if Cassie came along after Alma divorced him for other reason?
We can speculate all we want. The truth is, we don't know for sure.