I think people mistake the oft-referenced "rape isn't about sex but power" line to mean that it has nothing to do with sex, which is a very reductive understanding. Of course power is a big part of it, but if you're erect you're aroused. It doesn't matter if it's the power dynamic that revs the engine, what you do with that erection is what matters. There's an element of dominance in many consensual sexual encounters as well.
While I'm sure there are exceptions, these stories of frat guys and high school athletes hazing by sexually assaulting other boys are generally nothing more than latent homosexuality finding its only outlet in a culture that still vilifies homosexuality. Leaving aside the experiences of transexuals as I am too ignorant on that front to comment, if you have a dick and fuck a dude, you're gay or bi. Full stop. I think it's largely homophobia and/or denial that drives the other narrative. No straight man in the history of the universe said to himself, "I'm not the least bit sexually aroused by that guy over there, but I'm going to fuck him anyway just to show him who's boss."
Like so many other things, these types of assaults are merely the byproduct of a subset of our culture that still shames homosexuals and keeps some young men so deeply closeted they can't even admit who they are to themselves. Sports organizations and fraternities are probably the prime examples of groups creating this dynamic, which is why these stories tend to come from these organizations. Then of course there are certain churches that really drive this bus. It makes sense that this type of existence will eventually lead to socially unacceptable or violent behavior.