Loki: Since I'm coming out of lurk, I should probably introduce myself. "Hi, my name is Loki, and I have a big dick." (All, in unison: "Hi, Loki.") And with that AA-style greeting out of the way...
The racial issue has come up here several times, in many forms, and I think I have something worth contributing. (We'll find out for sure in a minute...) I'm starting with black men, because that's the stereotype I encounter the most; I'll get to the other ones later. (I myself have never encountered a stereotype about Greeks. But then, I've met remarkably few of them.)
* * *
"Where does that myth about black men come from?"
My theory only covers the US. I don't know if black men are rumored to be large elsewhere in the world, and having never been outside the US myself, I will carefully refrain from guessing. If black men are rumored to be tall on their backs in, say, Europe, a different explanation would have to apply.
Everybody here knows that before the civil war, black men had a tough time in this country -- I trust that aside from my penchant for understatement, I am not surprising anyone here when I point that out. What a lot of people fail to realize is that women didn't really have it that much better -- women were as much under the thumbs of their husbands (or sons, at times) as the slaves, and probably no less likely to be beaten. (Funny how history books leave things like that out.) Some women would respond to this sad fact by abusing their slaves more, desperate to prove to themselves that they are above someone in station.
On the other hand, some women would actually open up to another slave. The relationship between "free" white woman and enslaved black man would -- in theory -- be complex, and emotionally tangled... but Romeo and Juliet had that same problem, and they both got over it. And white woman and black man actually had quite a bit more in common than Romeo and Juliet did. They can relate to each other -- they're each under the same man's thumb. Combine this potential for mutual empathy with a shared resentment of said slaveholder, and you can probably picture a sexual relationship quite easily. Possibly romance, as well. The black man in that situation is certainly going to be much more sympathetic to her needs than the husband... partly because, unlike hubby, he doesn't suffer from the delusion that she is his personal property.
But... soon, inevitably, massuh finds out. What does he think?
"My wife strayed because I'm an insufferable bastard?" No. "My wife was more able to find love with a man who is supposed to be under her thumb than she was with me?" No. Both of these explanations require too much honesty on the part of the massuh -- he'd have to admit that if he was a better person, things might have happened differently. On the other hand, if he decides that his slave was better-hung than he is (whether it's true or not), that's something that can't POSSIBLY be blamed on him, because he's powerless to change it. And at the risk of generalization, it helps that black men do tend to be showers. (Us white guys are more accustomed to having to shrink when it gets cold.)
Repeat this chain of events on plantations all over the country... and soon, people start to believe it. And today, there are black men who spread this myth that was born from racism -- they even capitalize on it.
Pretty damned funny, if you're into irony writ large.
* * *
I have more thoughts on this subject, on other races, but this is my first post. I don't want to hit you guys with War & Peace after just barely having said hello for the first time.