I am a straight guy, and I can tell and appreciate the difference between an especially attractive man and an ugly or average looking one. And if I must see another male nude, I would much rather be looking at a good looking, in-shape guy than the opposite. (Disturbingly, it seems to me that it is the flabby, saggy, out-of-shape types who tend to roam around the gym locker room nude for minutes on end, requiring one to be very careful not to get a glimpse! Once, in grad school, I had the severe misfortune of seeing one of my professors showering at the university gym--the vision haunts me to this day!
As for seeing other penises, I do find it interesting for the sake of non-sexual comparison (and, yeah, I know it's bad of me, but it is fun to confirm that I really am the biggest in the locker room). There are a handful of men in the world who are just so dazzlingly handsome that I can look at them and say, "Damn, that is just a physically beautiful person to look at, regardless of sex." Brad Pitt would be one example. Another example is Michaelangelo's David--I had the pleasure of seeing the original sculpture a few years ago and was just captivated by its beauty. I was amazed at my response because I had seen pictures and reproductions before, but they never affected me like the original did. I'm not really an art-lover, but my hat was off to this masterpiece.
All in all though, if I had my druthers, I'd rather be looking at a nude woman!