Any type of taboo relationship has always been popular in any kind of story. It's good source of drama, etc, but its also for some reason incredibly hot to most people. Incest, Trans, student/teacher, young/old, feet, anal, boss/employee, master/slave, furry, or just straight up beastiality in some cases (MLP fans, im looking at you) and just about anything else you wouldn't want your mother to know about are all examples. If its wrong, someone is gonna find it hot.