Many white men don't like seeing their women with black men. But they don't reject them, they only reject them if they continue dating the black man they are with.
but yes that is the real meaning behind it, i've known it for a long time
it means, once the white person dates a black person, they are no longer accepted into the white community(they can never come back)
so yes that is what it means, this is an old saying
Nowadays some whites may tolerate it more, because these women are having children by these men, but they still dislike it. Many people mistake tolerate for acceptance