Ok...my psych degrees are about to come out in full ******* here, so hold on.
Societies have always gravitated toward groups of people in similar situations. It happens in neighborhoods, high schools, colleges, in the work place, etc... We have a natural tendency to spend time with people who can relate to our joys and miseries and failures and frustrations.
An example: in high school, athletes tend to befriend other athletes. Agricultural ******* befriend other Ag *******...and so on. The same exemplum can be seen at work among colleagues of different departments. While this isn't a set in stone rule, it is the norma.
I have found that the one anomaly regards the military. Black, white, hispanic, asian, etc...they all go through the same thing. In base housing we had neighbors of all races and creeds living around us. That commonality extended to all of us regardless of our backgrounds and created a bound that revolved around financial hardships, deployment stress, and so on.
To your original question...yes there are some white couples out there that maintain a relationship with black guys outside of the bedroom, but there has to be a social connection and commonality for that relationship to be successful. Think about it, you have to have something to talk about other than sex in order for a conversation to last.
I counsel young married couples in a similar way. Too often, after year 2 of a marriage, they are at each other's throats and come to me for advice. When we drill down to the heart of it, they almost always say the difference is that the crazy sex waned and they realized that they had nothing in common to talk about.
Personally, I dated a few black men before I met and married my husband. While I enjoyed those relationships, I never found the emotional or social connection that would satisfy my criteria for marriage. I did find those intangibles when I met my husband in 1991.
My $0.02 worth. Sorry for the long post.