Angry about dating someone of a different race

Why do some woman get angry when a man of their same race dates someone outside of it? My friend is white and she’s engaged to an African American. He’s a professional athlete so he has a lot of followers on his social media. Everytime he uploads a picture of her (she’s white/blonde), he gets multiple negative comments from African American girls.

Thank you ladies for all of your comments!! It raised a really great discussion and it was interesting to here different perspectives.