We define the world largely in terms of limits.
Zack developed sexually relatively early in life. That resulted in him feeling not just different but wrong in ways that didn't make sense. His inner world reflected that, and it attached a lot of uncertainty to his perceptions about himself and the world. His inner self has grown to expect uncertainty, and he put up a wall around it to control that as much as possible. Zack therefore thirsts for certainty: he'll be fulfilled by a small number of certain things, and he would hold tight to those that he finds, for example, a single person who accepts him and loves him. Zack's limits are largely about himself.
Braden developed later in life. It made him feel different but not wrong. He's initially unsure about his urges and where he fits into the world, but he's curious and unafraid to find out. His inner self relates mostly to that world, and he'll be happiest exploring it. Braden's limits are largely about the world around him.
The story as told culminates perfectly. Braden reaches through Zack's wall, and Zack lets him in. The two young men fall in love for the first time, and it completely fulfills both their young personalities as they currently are. It's their glorious dawn. They will continue to explore one another and all the different aspects of young love, and they will be intensely happy.
For maybe a year. They still see the world very differently, and it's doubtful they can ever really understand one another. As they grow in their disparate ways, they will tend to grow apart. Zack's needs will move him to keep solidifying his precious few certainties that he derives his strength from, there inside his safely developing inner world. However, Braden will want to test his limits in the outside world.
In both the respectful and the pervy paths, Braden was not fully open with Zack. The singlet was pictured in his high school photo in both cases, so Braden manipulated things. He got caught in the pervy path so he confessed - but only up to a point: he insisted they not talk about it again. In the respectful path he kept it completely hidden. (And by the way, the only eyebrows in the Straight!? universe that didn't elevate at the "he was just kneeling down to pick up the towel" line were the ones on Zack's face.)
None of that means Braden is bad, but he is retaining something important to him, and he's keeping it that way because Zack isn't in it. it's part of Braden's relatively worldly quest to figure out the world. Thus there's a serious gap in how Zack and Braden see their relationship.
It's very likely that the fully awakening Braden is going to go places Zack can't or won't or isn't going to go. It almost certainly means other guys. Braden wants to experience everything a gorgeous, curious, athletic, hung, horny, young blond guy with good funds and a dazzlingly unique curve in his cock can experience. There'll be no-o-o shortage of interested parties. Jesus. How far Braden takes that is hard to say, but it will get wild. That's the "phase" he needs to go through.
If Braden goes that way, it will separate him and Zack in both their inner worlds and the outer one. It won't just deviate from what Zack wants and needs; it will annul it. Zack's certainty will be destroyed, and part of his heart will be ripped out, with a scarred crater in its place for the rest of his life.
But that's how fucking life is. Those experiences force people to get realer. They redefine their limits to a more realistic place. And Braden will suffer from it too, because his life view will be getting reshaped just as much. Not only will he always miss his glorious dawn with Zack and the promise it seemed to embody, he'll go on to discover the actual limits of experience in the outer world, and that will inevitably entail harsh realities.
So they'll both grow and become more complex and hopefully better people. But they won't be together. Zack will continue his studies and work at college, and probably he'll hook up with Mikhail. That would be great; they certainly have a much better innate understanding of each other than Zack and Braden have. Braden may take a 2-year degree and then help run his family's lucrative business, and he may eventually own it. Then when Ash returns from his military stint, who knows? Ash and Braden also have a lot more naturally in common than Braden and Zack. Lord knows, Ash would be more open to worldly experiences than Zack.
Despite my thinking this to be realistic, I would hope I'm full of shit. My imagination really wants Zack and Braden to stay together and deepen their love for a long, happy haul. There were signs that could happen: the swimming pool session touched something all the way down in Braden; and he did broach the topic of having children with Zack (which was one of the sweetest things in the whole story.) Both those would support a lasting commitment. BUT - for that to actually come to pass, Braden will need open up about his other needs and perspectives to Zack, and Zack will have to open up to accommodate them. They'll have to work some hard stuff out.
Arryn, these guys are you. In writing and publishing their story you have let people know you intimately, almost like making love would. My heart goes out to you, and I hope you enjoy a long, happy story ending yourself. Best regards and good luck with life.