I'll never get that. I don't think it means they're closet
> gays, but I guess it's the 'ultimate' insult to a heterosexual
> man, so it's used for that purpose?
Classically, it's the insecure ones who feel disturbed by a third party seeming to endorse their own doubts about themselves. They often feel -- as they are, by definition, imperfectly informed on what a "real man" would/should feel when called in such a fashion
-- thrown off balance and react as if the challenge was a violent attack. The secure ones can laugh it off, being confident within themselves that the "charge" is BS.
Or something. ;-)