Cindy being a toxic, troublesome unappealing character and love interest is itself the appeal (and there are some meager positive traits of her character that allow to hope for fixing her). You sacrifice your long-term sanity, friends and stability for the corrupt, forbidden taboo. In RL you (usually) don't go for your friends hot girlfriend, but you may just subconciously think about it for two seconds and imagine how wild and crazy it would be, before you come back to reason and responsibility. This game allows to throw this all away and to live out that fantasy and gratification, hence the simple concept of Cindy ended up as the highest voted option.
Gillian also got many votes and her content is currently toxic as well.