A 'racial stereotype' it might well be but I wonder if anyone has ever done any serious research into the relative sizes of penises across different racial groups?
After all, there have been several studies which suggest that people of African descent might be genetically pre-disposed to have better muscle configurations for certain athletics events and it's obvious for all to see that there are other physical differences between certain races (such as the shape of their eyes or the thickness of their lips).
So perhaps the average size of what's between a black guy's legs might genuinely be bigger than that of what a typical white man has? (Alternatively, perhaps there's a bigger range of sizes for black men, even though the average might be the same, meaning that both the biggest and the smallest penises are to be found amongst men of African descent?)
All I know for sure is that when I used to get changed in the referees' changing room on a Sunday morning in Sheffield's Concord Park, there was one fellow referee whose 'manhood' always drew comments from others after he'd departed (because it was about three times the size of anyone else's) and he was the only black guy there:
http://i.dailymail.co.uk/i/pix/2013/06/26/article-2348753-004DD2F600000258-361_634x513.jpg