This is one of those times when experience and exposure make all the difference. As an "older guy" (a term I don't particularly like getting used to, but one which applies since I'm old enough to be your father), I know I've seen some young men your age at the gym sporting wood before and thought little of it--except for occasionally being impressed. An erection, in and of itself, is not a come on. I cannot imagine someone being offended by one either.

In short, as you shower there more, you'll get over the unwanted hard-on for the most part. If you get an erection, just ingnore it and finish the shower. Personally, I think the guys who overly cover up with the towel to a from the shower and always stand with their backs to everyone are a bit strange. It depends on the locker room, though. At my gym, rarely does a guy wrap himself in a towel. Those who do and who are overly "modest" stand out. The ones sans towel are in every stage of arousal.

BTW, frankly, I'd be more embarrassed about a tiny dick that barely hangs in the locker room than I would be about one that is impressive like yours is. Don't worry it and enjoy.