Generally a fear of locker room nudity boils down to 1 of 2 issues: fear of being judged (low self-esteem or body image), or fear of someone "losing control" at the sight of you (homophobia).

So you either are afraid of someone sexually assaulting you, or you think your penis is tiny. Either way, here's a secret: No one cares. Just don't go swinging your junk in someone's face.