Race-Mixing isnt sinful per-se, but it is definitely contrary to the Natural Law (which means it could be sinful in some and likely most cases). Most Christians arent aware that the Natural Law is a thing because Liberalism has replaced actual Christianity (which essentially denies the natural law and presumes that Man is solely a reasoning creature without an Animal Nature, when in fact Man possesses both an Animal and a Reasoning Nature according to the ancients). Some things may not be sinful in and of themselves, but are 100% disordered according to nature and therefore deleterious to ones health.