I have to say that I am surprised how strongly the feelings run about whether or not someone keeps his towel on or takes his towel off in the men's locker room. I am one who remains perplexed by the seemingly obsessive covering by some men with their towels in a setting where nudity is the norm, and the consternation that many of them seem to express as they try to keep their privates shielded from view distresses me. I don't particularly want to see their junk, but I do worry about the stress they put on themselves to keep it covered. I guess I figure that in a locker room, people are sometimes (perhaps often) naked, and if nudity freaked me out (my own or someone else's), then I'd probably find a different setting to change. I don't get why these guys are so upset by seeing some other man's genitalia, and I figure they can look away if they have chosen to put themselves in the setting where they have a good chance of seeing just that.
But the OP addressed behavior that is really stepping over the line, I think. For someone to tell a guy to put on a towel when he is changing clothes in a locker room is incredibly rude. It reminds me of the Emily Post question where a writer asks Ms. Post about what to do about someone ELSE'S perceived rude behavior at the table. Ms. Post remarked that it was far greater an offense against civility and etiquette to POINT OUT someone's perceived faulty behavior than it was for the person to err in a point of etiquette.
I'm not saying the OP was wrong for changing sans towel. Far from it. I am saying that the guy saying something about it was just plain rude.
One last comment...as for the numerous posts about boys raised by single mothers, I do want to say this. In discussions with my wife, I ascertained that the way that women act in locker rooms (generalizing here, of course) is significantly different than how men act (or have acted traditionally). My wife was shocked that men walked around openly nude in the locker room when I told her, and she admitted that she would never have even imagined that was the case. Had my boys been raised by just her, there is a good chance that they would have a different view of nudity in such places than they do as a result of my influence.
Numerous experiences changing in locker rooms at swimming pools, campgrounds, and the like while on vacations have taught my sons that it's NO BIG DEAL to be naked in a men's locker room. I helped condition them to understand that it was normal behavior; consequently, they think nothing of it. I'm guessing that if I'd not been there to communicate that concept, and if they had been raised only by their mother, they would perceive that differently. I don't see that as a criticism of women, but as an acknowledgement that, in many ways, males and females see the world differently. It helps boys growing up to have a male influence around so he can get a better understanding of that male world view.
Oh well, that's my two cents, for what they're worth.