Its not *promoted* anywhere and its contrary to nature, but its also not sinful if the other race is Christian. But 99% of the time, interracial marriages are sinful because the other race isnt Christian.
Im not saying that miscegenation is a *good* thing (in fact, believe its a very bad thing because of how it denies the children their heritage), but if you arent an ancient israelite, its technically not sinful, though maybe theres some church teaching that was hidden in V2 that says it is. Of course, mixing with a non-Christian is *always* sinful, regardless of the race, but race-mixing among Christians isnt a sin, its so contrary to nature that the Church has never encouraged it and no one in their right mind would race mix anyway.