It all started when the war between the demons and werewolves broke out where both parties unintentionally interfered with the human world. Two different creatures in one disparate world---of course, there would be miscellaneous traits formed by these life forms.

For some reason, it would be a great misery if immortal creatures would mate to a human, especially if those creatures are werewolves, demons, vampires, and such. Heating, mating, and other specific reason for ruining one's fate---it's not applicable for their laws, rather, not destiny nor fate. Different life forms shouldn't be mating with humans since none of them are fated to have a pleasant life.