I think it's a mixture of truth and false perception. There might be some real instances where a white girlfriend cheats with a black man, but it's blown out of proportion. It's important to remember that we are all individuals with our own choices and behaviors. Race should not be the main factor in determining if someone is cheating or not. We should focus more on the integrity of the individual in the relationship rather than making assumptions based on race.
In my opinion, these so - called stories are often over - hyped. There's no real evidence to suggest that white girlfriends are more likely to cheat with black men compared to any other group. It could be that in some communities, interracial relationships are more noticeable and any relationship that starts during an existing one (which might be misinterpreted as cheating) gets more attention because of the racial factor. But in reality, cheating has nothing to do with race but more with personal values and relationship dynamics.