How can a white swan fall in love with a gray crow?

White swans don't like gray crows. In the fairy tale Ugly Duckling, the white swan is regarded as a symbol of beauty, purity and nobility, while the gray crow is regarded as an ugly, ominous and humble symbol. "How can a white swan fall in love with a gray crow" means that a white swan will never fall in love with a gray crow, which expresses people's inherent cognition and prejudice about beauty and ugliness.