I'm quite sure it's not the junk. It's more, like, the belief that their junk should prevent them from doing it, and therefore if they do it, they will appear to others as if they don't have much of it. And men tend to dislike it a lot.
Seriously, unless the person is excessively fat and/or muscular, there's obviously room for the junk to move away and avoid being squashed. I can't imagine I couldn't do it even if my stuff's size tripled or quadrupled. There's more than enough extra skin for it to go underneath or over it.
No. Most men don't do it because it's "gay".