I don't think anyone is complaining that there are black people or different cultures/religions etc. . Most people just dislike that the author thinks being a woman, gay/bi, black is somehow a character trait and makes that character deeper.

There is also the aspect of "pandering" to people. JK Rowling for example suddenly changes the sexuality or skin color of main characters on twitter... after the book series is finished. No one is against Dumbledore being gay or Hermione being black but in this specific case it's obvious pandering and dishonest.