i think the biggest part of it is the two participants. Perhaps these two dudes have already "done it all and seen it all" maybe grew up as pre-teens and partipated on the same sports teams, seen each other naked all their lives, and skinny dipped with a group of guys and girls... MEANING that they've grown past the point that they could possibly find each other attractive... and I'd add "anymore..." because I'd venture to say that all relationships go upwards and then plateau or find a natural stopping point- a comfortable place where 2 friends can comfortably exist as buddies who are chill with each other in that way.
in that case, I can see the event basically being pretty utilitarian- and both guys would have their energies directed only at the porn playing in the background. Maybe what- 5% of the time?
HOWEVER I think that any shared experience that happens between ANYONE of any gender arrangement involves the shared interactions between participants. This is Man/woman, woman/woman, Man/man, strangers, or intimates. Like the saying goes- "it takes two." You have a living human being next to you, as compared to a 2 dimensional representation on the screen in front of you. What are you, made of stone? Hello. Humans are drawn to social interaction- to watching each other and running a mental commentary about the other person- thinking that you are interested in them, wanting to be like them, wanting to GET IT ON with them, or being repulsed by them. We're not talking about 2 ken dolls sitting next to each other with no visible bulge to be seen- this is REAL LIFE with sexual behavior being played out right in front of you. Don't be embarrassed to say that you're gonna peek over, watch- stare, or maybe even lend a hand. Sheesh.
This means that the other 95% of the time that the two guys are going to have some sort of sharing of energy between them during the act. Otherwise, it's like ignoring that the other person is in the room with you- and that is just downright rude. Show me any guy who can rub one out while mentally willing a brick wall between him and the other shlong getting some palm love, and I'll show you a badass dude so emotionally hardened that I'd think that dude would scare the hell out of the other guy- not even the totally friendless would want to be his buddy or hang around him at all. Can you imagine having the mental discipline to totally ignore a person, focus on sole stimuli in front of you, run your mental imagery and actively engage in erotic thoughts, and sustain an erection to the point of a wet sloppy orgasm? REALLY? WTF.
Now to qualify: what that interaction means is totally different with every guy- and it depends on both players. If the guys are militantly opposed to any sort of 'homo' scene, I doubt it would actually happen to begin with. If both guys go into it and simply enjoy the scene, and be OK with enjoying *themselves* first and being relaxed with allowing another dude in the room during that special alone time- yeah it can happen. It takes a while to get guys on the same page who would be comfortable in that way, and guys who are self confident about themselves and fully comfortable about with who they are, and courageous and fully willing to take the risk to be vulnerable with the other guy, opening himself up to that judgement... instead of just waiting until his next shower time.
Labels are pointless and stupid. If this thread is indeed about "how many straight guys" and is meant to be 100% 100% 100% as others insist on... yeah, probably a bit more on the 'rare' side- just because two alpha dogs are shutting each other down. But if you step the f()ck down from the top ladder rung there's definitely room for negotiation.
And if something pops up, damn it, box the damn clown- regardless of the audience.