He got horny in the locker room and shower, he could not help it.  It was the smells of his teammates, sweat and testosterone and the sight of their naked, muscled bodies. When he was younger, he used to try and hide his boner, but then he realized that he was impressively hung, and there was no reason to be shy about it. All guys got hard-on’s and his was bigger than most, so why not show it off.  

Displaying his engorged meat also attracted the various locker room fags and curious straight guys.  So it enabled him to get sucked and occasionally bend over another dude and fuck them hard.  

There really is no reason not to display his stiffy proudly.