Why do we pretend that we don't check each other out at the urinals?

So, you're at the urinal and the guy next to you quickly looks down at your junk then looks away. Ok, it happens all the time so why do we pretend that we weren't looking? I'm not saying you should stare and be up front about it, just think it's funny you don't think I saw it. Have you guys ever noticed when you walk into the restroom, a guy at the urinal will turn his body away so you can only see his back? It's like he expects someone will try to look at it? I get a laugh at that. I can't say I check dudes out at the urinal but I'm sure I did it out of curiousity and forgot I did it. What about you guys? Be honest. It's not about being gay. I think most guys have done it once or twice, straight or gay. Lets be real here.