It's about 2 things: comparison & just simple respect/admiration for the human body.
I mean, if a guy is into working out and sees another guy with chiseled pecs and huge biceps, he's gonna think, "Dang, that's impressive. Good on him. I want to look like that one day," which is totally different sentiment than, "Oh, I want him to hold me in his big, strong arms."
Whether it's a dick, a pair of boobs, a butt, shoulders or calf muscles, an impressive specimen of anything is still going to be impressive, regardless of who you are or how your sexuality leans. In the case of things that aren't particularly impressive, the curiosity is still there because, well, you don't know until you look.
I agree with all this but think there is more to it than just that. It is a competitive man thing for several reasons.
My dad has a good friend who is into old muscle cars, the first thing he wants to do when he sees one is look under the hood.
I think it is a lot like that for many guys. I want to know what my competition is like in more areas than just on the mat. It is more than just who can show off a bit and get the hottest chic n school. I want to know that if by some chance I get beat on the mat or on the field can I feel confident enough to strut right up beside him in the locker room showers, smack his ass or even his shoulders and say good match or good game and know when he looks over I will be competitive in that aspect.
I also think there is something athletic in nature about our dick and balls. when I started playing baseball the first thing my dad did was take me out and make me buy a cup and jockstrap to protect my goods as he said. This was before buying a helmet, mouthpiece or anything. It didn't matter if I was going to play outfield, pitch, catch, short stop or third base I knew to wear my cup and protect the part of me that my body would rely on to develop not only confidence but the muscle and stamina to become the elite athlete I wanted to be.
When me and 2 of my best friends growing up decided in 7th grade we wanted to join the freestyle wrestling club, my dad again took all of us to the store to buys jockstraps, he also told us the he would insist we shower after our practices and matches and ask if we understood that the uniform we would be wearing would not be real friendly to our somewhat uncontrollable adolescent dicks and that our friends and classmates would pretty much be able to follow our development if they chose to attend the matches.
Much like the engine in a muscle car is the key to how much fun the owner will have with it. Our dicks and balls are the plug in to the best of being a guy. And it can become sexual in ways other than wanting another guys dick up your ass or anything like that.
Ever been in the showers when one of the guys gets a boner, just wondering what charged his blood flow is enough to charge your own sometimes. My dad told me about a guy in his pe class who purposely got a hard on just to see how many other would follow suit. In the thousands of open showers I've taken I have been on both sides of that situation (not on purpose) and have had it happen to me.
My roommate brings a chic back and starts banging her, just hearing him moan and breath hard and knowing the pleasure his dick is bringing him can get me pretty worked up.
Many of our private team discussions, especially when younger centered around those same parts of our body--our dick and our nuts--how do you position it when wearing a singlet? do you worry more about comfort or how it looks? how long before a match do you try to jack off so it doesn't affect performance but limits your chances of getting a badly timed boner? No other part of our body is worried about among athletes more than these 2 are.
In the realm of male athletes I have seen short ones, tall ones, fat ones, skinny ones, ones missing an arm, leg, foot, hand, ear etc, but I have never seen a male athlete without a dick and at least one nut.
Ok, sorry again for the long-windedness.