Most of the men have always been undressed in the changing rooms so it is not issue for them. The problem is that if you are not used to being nude with other guys it will erect so you must learn not to link being nude with sex. Or after some time of touching your dick while washing in the shower (as you need to put soap everywhere) it can begin to get hard. Even if you are not gay it is possible that in locker room as soon as you get naked your dick will start to erect. Those who are going to attend sport gym for a first time face a locker room cloths changing before the training and of course after when the training is finished.
Sportsmen changing clothes in the locker rooms