It is taboo to be attracted to your relatives.
Dan is sitting beside his cousin Lisa on a living room couch.
Dan wants to express that Dan thinks Lisa is good looking.
Dan tells Lisa that Lisa has a strong fashion sense and chooses flattering outfits.
Dan admits to Lisa that Lisa is exactly the type of woman Dan would like to have sex with.
Lisa appreciates the compliment and feels validated that her unique style is noticed.
Lisa feels violated because Dan is suggesting an act that will pollute potential offspring's gene pool.