People believe all kinds of ignorant shit. I've heard of people say that black men will stretch a girl's vagina out, or something stupid like that. Part of me thinks that's crap and they're just being racist fucks. They'll have sex with her, but they can't be serious about her because she's been with a black guy, so she's a slut/damaged goods.
Don't forget "they have AIDs" (actually have a friend who heard this one recently).