Ruby, yes that is interesting aspect that the vampire, demon, etc. might have a different agenda that might involve sex or but possibly mixed in with other urges, possibly to take the person's blood, soul, etc. They might have to fight the urge to eat the person they are coming to love. That kind of creature is natually a good metaphor for people into BDSM, as to the vanilla community their desires for what they would like to do to those they meet can be perceived as 'evil' and certainly alien.