just that its a myth perpetuated by the porn industry to cater to certain "crowds". 
Dude, they only hire "big" dudes.
but to always look at a black dude's junk (in clothing) and ALWAYS make a comment, shows your insecurity of your own shit.
Ain't anybody looking at his junk until you called attention to it. I think you need to consult Onemorerep or Bay.
Furthermore it's not a "myth" that started with the porn industry...