Human behavior usually consists of rational justification of limbic instincts. A powerful instinct of all males is to protect their women from outsiders. People of other colors are outsiders. In slave days, black men were furious that they could not prevent sexual access to their women by white men. Today, the role is being reversed.
Black guys know it, and revel in it.
White men don't like to admit their resentment because to do so underlines their inability to do anything about it. It's more than dating. It's potentially explosive sexual competition. Instincts involving sex aren't always obvious, but they are there, and powerful. Yet if my daughter told me she planned to marry a Japanese boy of impeccable credentials, I'd nonetheless feel a visceral resentment.