The prohibitions of the O.T. were for the Jєωs to preserve the true religion and the pure blood-line that there be 100% proof of Our Lord's identity. The N.T. retains one of those prohibitions, to preserve the true religion. There is no longer a spiritual reason to prohibit race mixing. The prohibition on marriage to those of false religion, however, remains. As those who practice the true religion grow ever scarcer, Catholics should be proportionately more concerned with gaining true converts, and with providing young people of marriagable age with a pool of suitable candidates.
It seems that neither is the case. Those with the true faith spend more energy on fighting one another than on making true converts. As for facilitating suitable partners for the young, they are too often left with the miserable choice between "conversion dating" or life-long single-hood.
Bp. W.'s point is a practical, not spiritual. A mixed-race couple will encounter many difficulties of a social nature, as will their children. These difficulties are not insurrmountable, whereas the problem resulting from a marriage of mixed religion has eternal consequences.