I think that, whether or not a guy admits it (and many won't), to some degree most guys enjoy looking at another man's hard cock . . . In hetero adult films, the guys' big cocks are as much a part of the entertainment as the woman. And aren't guys often curious?
Sure, in adult films much of the turn-on is, for example, watching a woman suck that big cock, or seeing that guy give a woman or women a good fucking. And women who watch porn surely enjoy seeing a big cock. But let's face it, more guys watch porn than women. And doesn't everybody enjoy seeing a guy shoot a big fucking load? Besides "proving" that the people are really fucking, I think that cumshots are a regular part of porn because guys just love to watch a guy's cock as it explodes and shoots ribbons of cum.
As for me, my orientation had always been hetero, but I've enjoyed it when guys take long looks at my cock at the nude beach. It always makes me a bit excited in fact. Some of the guys are obviously gay and oh yeah it's made me think about being with the right guy . . . perhaps in an m-m-f threesome or one-on-one.
So when the opportunity presented itself, last May I jacked off with a guy in a sauna while I was on business travel. It was totally anonymous and hot as hell. Yeah, it was a turn-on for me to watch the guy stroke his thick cock and caress his balls. His cock looked totally different from mine and I enjoyed watching him. And fuck if I didn't go wild when the guy began to caress my smooth balls and stroke my cock, which had become extremely hard . . .
And I enjoy it now when I'm in the steamroom where I work out and sense that a guy wants to jack off with me. Part of the turn-on is seeing the guy work his hot cock. And I've enjoyed it when a guy sucks my cock while jacking himself off.
I've always been "straight," but what I've cum to enjoy isn't exactly heterosexual and I imagine that many "straight" guys would enjoy this sort of experience if only they'd just let it happen.