Because men tend to be territorial with their women. White men don't like seeing quality white women with black men, and the same with black men not wanting to see quality black women with white men. At the same time, they might think it's ok if they want to smash and dash on a chick that's outside their race, but wouldn't marry.
So yes, there is some hypocrisy there, but at the same time there is a bit if gender based entitlement, in which men think they have a privilege that women shouldn't have, i.e. dating/marrying/fukking out.