Just because a dude is aroused by seeing a stiff dick in no way indicates he's gay - or even bisexual. If you think about straight porn: What does the camera home in on, primarily focus on? The man's cock. When it's time for the orgasm: What does the camera frame? The man's cock as he strokes it to the big finish, which is often referred to as "The Money Shot," so we can watch every blast of baby batter as he shoots and sprays that sweet nectar everywhere. Guys love - I mean, love - watching another man blow his load.
Why? It's called "transferrance." When we see images, our brains are wired to empathize with what we see, we relate to what the person is doing or feeling. It's why we connect with and movie or television show we like. Men mentally place themselves into the situation; they know the sensations the guy on that screen is feeling, they can appreciate it. In straight or bisexual porn the hetero man gets visuals, but in gay porn the images are all about men enjoying their cocks; we get to see cock in damn near every frame.
We get to see and relate to the hardening of it, the stroking of it, the elevating of the balls, and, finally, the sensations of the spasms and pulsations in it when the guy cums, which can curl the toes and we know to be such an awesome experience.
So, all you straight guys out there, go ahead, enjoy the surfing of all those hard-ons spewing those huge loads of jizz. And do it without any sense of guilt. It doesn't mean you're gay, or bisexual, it means your brain is wired to relate to it and empathize and feel a kinship. It's what makes us all brothers.