You're talking about a physiological response to an arousing stimulus. What is arousing can be cultural, but heterosexuals are heterosexuals whether they live in Antarctica or Burkina Faso. Penises are not arousing to straight men, regardless of culture, the population observing the penis, or any other factor. It's possible that there may be some men who are bi with a strong heterosexual bias and have a latent reponse to penises, but I'm extremely sceptical that this is as widespread as you seem to be hinting, unless I've misinterpreted your post.
I've never read about, heard about or experienced anything remotely like this level of widespread heterosexual atraction to penises anywhere. The only place I've seen this idea that straight men are all secretly just waiting to be "turned", or have latent heterosexual responses, is in the claims of gay members on LPSG. I wouldn't be surprised if a strong element of fantasy or wishful thinking is involved.
I understand what you mean. However, I do think penis size, at least, should not be underrepresented as a source of excitement (perhaps not purely sexual, but tapping into the man-ego) to men, especially if you look at porn in general, when anything short of real-life huge usually just isn't quite good enough.
The incidents I'm talking about have happened a few times, perhaps a dozen times or so in 3 years of being around naked people 3-6 times a week (times 2). So these are not impossibly high numbers either.
The most recent and probably most striking incident was last summer, when i was changing and taking a shower before going to the pool. There were a lot of children and teenagers, including one guy, probably 16-18. He was tall and had the best body of his buddies, and was proudly displaying his sort of physical superiority status, but when I arrived, he first got a slight semi to seem big, which he really was, but I out-sized him and perhaps his attempt at pushing a semi went too far, but he got an erection. Later I was waiting outside and he came out with his girlfriend. He saw me looking at him and her, and he sort of turned her around and lead the way out.
It was weird, but it definitively happened. I told my girlfriend as soon as she came out. She high-fived me.