Given, from what I remember, humans (and most other animals) are mostly innately set to find the scent/pheromones of others in their direct bloodline unattractive, maybe they've got a malfunctioning/absolutely fucked up version of this?It can only get worse if they do try to share with their family. Imagine having a sibling saying they find you so attractive that they want to have sex with you.
Perhaps it has some Freudian nature or something. After all, you have some people wanting to have sex with their sister despite the fact that they are related to said sister.
Or maybe they're just screwed in the head..