Regarding white women "fantasizing" about black men;
I believe that many white women date black men as a form of rebellion in response their white parents/family/communities who, actively condemn interracial dating and/or marriage. Growing up in a small town with a population of 5,000, I knew a couple of white girls who dated the black boys in my school. They were all from affluent families, and I'm sure that mom and dad did not approve of the swirl. (Allowances got cut and cars got taken away).
Another reason why white women fantasize about black men could be from their desire to leave white culture and assimilate into African-American culture
because we all know that acting/looking/listening black is 'cool'. Hell, Just look at Iggy Azelea and the majority of the Kardashians. (I would've posted that Cash Me Ousside Chick, but she aint 18 yet

)
Lastly, (echoing my previous post) white women may just 'fantasize' about black men due to the "Mandingo Myth" of black men having bigger dicks and what not. Most of which, is just not true. I've dated many black men, and I don't buy into the hype of them being better in bed . . . nor the bigger dick. As a matter of fact, the biggest dick I've seen was on an 80 year old white dude . . . that Goddamn Viagra!