In my country, there's a genre we called "Beauty and the Beast", probably mostly used by erotica story. The genre is used to refer a relationship (sexual obviously since it's a sexual story), between a beautiful young(sometimes rich) girl, and an ugly old man(usually poor and dirty, or work as a low blue collar worker like street cleaner or trash gatherer). I guess the arousal came from the difference between those couple, from age, appearance and status(or even caste in some story I've read). That difference become some kind of social taboo which, for some people become a kink/fetish in my opinion. Just like step sis or step mom kink/fetish probably, those relationship can be seen as taboo too right?