Both can be supernatural, can be turned, can be mated, one human/one a supernatural type, different supernatural types and the forbidden aspect of it, become a supernatural through experimentation.

Werewolves (born, turned, rival packs, mates)
Other types of shifters (were: tigers, lions, bears, foxes, different types of cats)
Mermaids
Fairies
Banshees
Kitsunes / Nogitsunes
Elves
Vampires (sires, ancient vampires, newly turned, sire/created etc)
Dryads
Succubus/Incubus
Dragons
Sirens
Witches/Wizards/Warlocks