The only term I could think of is defeminization.
It seems like most people understand what emasculation is and the things that cause it.
What are some examples of situations that can cause the feeling?
Edit: changed effeminization to defeminization.
A flat-chested female friend of mine was bullied by her female peers as a teen; they called her “A-cup”, especially around boys.
I’ve wondered about this before. Given my experience as a male teenager, I do not know how I would have coped with an aspect of puberty being so prominently visible and literally front and center. Males obviously have the whole penis size thing, but its an organ entirely out of sight in public. Your story sounds awful!
That’s a particular brand of cruel.