It doesn't happen very often. At the most I'll get a semi if I see something or someone that arouses me but I've mastered the art of hurrying to the shower and stepping under a cold stream of water to deflate my cock if I felt it was continuing to get erect. There have been rare times where I couldn't get into a stall fast enough and had to just accept that it would get harder and people might see it. The last time it happened it was when myself and another guy were both standing at our adjacent lockers preparing to head to the showers. We were both removing our clothes and talking about nothing. I'd seen this guy many times on the gym floor but never had I seen him nude. He dropped his shorts and out flopped a big hairy chode of a cock and a beautiful set of egg sized pendulous balls that were covered in chestnut colored hair. His cock sat perched atop his balls and almost seemed to be staring me in the face. By this point I was already beginning to feel myself get erect beneath my towel. He seemed to deliberately give his dick and balls a tug as he slightly raised one leg to get a good hold. This action set me over the edge and I pitched a tent right there before his eyes. He looked but didn't say anything to indicate that he saw the bulge that was noticeable beneath my towel. He smiled and headed off to the showers and told me he'd catch up with me next time we were at the gym. A few guys came into the lockerroom but nobody seemed to notice or care that I was erect beneath my towel. Had I been naked it would have probably been noticed much more.