it's a sign of insecurity. it actually makes me sick a little because these men aren't acting like men when they do that, they're acting like spineless cowards that are afraid and they have a subconscious problem. if you feel like your "woman" would leave you simply because another black man made an advance at her then you should consider the strength of your own relationship and the basis of your attraction to her (and her to you). to me, it can illustrate how there's no real relationship occurring other than a physically based one, one that revolves around "race" and attraction to that aspect (one where you feel like you have something special just because she's white, not based on anything else). and if the woman is open to other advances (more specifically from other men that share the same race as you do) then that shows how shallow her feelings are for you.
if that's the case, she could be with almost any black man and you'd have nothing special to offer to her. and as for the black man, he holds his white woman sort of as a trophy, feeling like he's achieved something because he has a white woman in his arms...