[Z]

I think part of it is because as others have touched on, if you're not blatantly flamboyant, how do you suss out who's gay and who isn't? Of course, some people innately know in other ways and others learn it over time but I think generally it's a learning curve and most gay men tone it down once they've been out the closet for a while and realise that you don't have to be a stereotype to be noticed.