Humans (since they are getting more rare from being slaughtered by demons, supernatural beings and such) are being sold off at auctions. They are fed and 'trained' by which means that some are turned into real sluts while some remain 'pure'. (18) Mark is on sale as a pure pet, he is still a Virgin and has not been introduced into any kinks or sex toys, anything. (16) (He is a human) Dark is a real slut where he has been fucked countless times by other demons and knows mostly all the kinks and toys.
Anti and Jack happen to walk into one of the auctions, wanting a pet each to call their own.