You're probably used to laughing at hypocritical religious types, like the proprietors of a bakery who refused to serve gay couples, but had no problem celebrating dog weddings. Apparently, Jesus' final commandment was "Love one another as I have loved you, and as my two dogs Buster and Miss Thang have loved each other since their wedding day. Amen."
But at least that bakery was taking a stand about a real religious tenet, and not a sliver of pop culture. Unless we've decided fandoms are religions now, no one should be arguing that a character's race, sex, hair color, or anything is sacred. They're FICTIONAL CHARACTERS.
Just because her name is Mary doesn't mean you should worship her as such.
A lot of people are going to tell you that their biggest problem with the alleged Mary Jane casting choice in the new Spider-Man movie isn't about race, but simply that the character has to be a redhead. And yet these same comic experts aren't going to have a problem with how Mary Jane and Peter should already be married in the Civil War timeline, or that they are supposed to meet in college and not high school, or anything that has to do with her internal character or plotline. Forget everything else; it's the hair color that's sacred.
"Oh, what does he know?!"