Actually, no I don't. Consider an interest of yours - any interest (rebuilding cars, woodworking, whatever). Now, don't you have a circle of friends that share your interest?
Her husband has a relative (brother, I think) who has a white girlfriend. Then from her, and others, her circle of friends that are interested in Black men expanded, much like your interest expanded to include like-minded friends. Although we've never met in person, I like to consider myself one of those friends as well.
It's like in this forum. You see so many people interested in interracial relationships. While you probably don't know anyone personally, it's easy to believe that this is a very widespread phenomenon, while it probably isn't in mainstream society. But it is catching on, rapidly! It's getting so that I rarely go anywhere with my husband without seeing at least one interracial couple. And I'm sure you noticed the same.