You see this happen in the dating scene because there is a racial construct established in society with whites being placed at the top and blacks being placed at the bottom.
The best way to explain it is if you treat it like a currency. Not all currencies hold the same weight in value, with certain currencies being percieved as either being higher in value or lower.
For example:
Take a pound and put it against the dollar. The person with that pound (£1) will end up receiving more dollars ($1.23) then they had with the pound as the perception is greater in value. Now take a dollar ($1) and put it up against the pound and you'll be getting less then what you had with the dollar (£0.81)
The same applies to race. Because whites are seen as better/more value they can date out and get more then what they could if they dated other white people (date a person of colour who is more attractive, has money, higher status to themselves).
whereas a black person who has these traits (attractive looking, money, status etc) and dates out more times then not will end up with a white person (or non-black) that's not there equal/equally yoked
Think of the old average/ugly looking white dudes who date asian/black women that half his age, slim and attractive. (we've all seen it)
They have the percieved weight/value to do this which they could never do with white women.. or think of serena williams who married a men who according to the internet has only 10% of her wealth or the guy from Ghost who is married to the most average looking of white women when he could have attracted a black woman who had twice the looks compared his wife.
It's all around us and i see it on a daily basis... not just with whites but with other groups too. It's why i never get the hype with interracial dating because more times then not we're usually dating down to what we could get.