Men are ashamed of being naked before another man, why?

..... because they know that there is sexuality between them. Even when the society, and they themselves deny it, and live as if sexuality/ eroticism between them does not exist, they are subconsciously highly aware of it. But the society expects them not to be shy of getting naked in front of guys because, it claims that there are no sexual feelings involved.

Notice that the guy stripping the junior is jokingly called 'gay' by the person who posted the video. Men don't seem to mind it if this labelling doesn't threaten their membership of the men's space. It becomes an issue only when this label is used in a 'heterosexual' space, where it threatens a man's membership of that space (men's spaces are converted into heterosexual spaces in the west).