You say "tumblr" and immediately it's like "Yeaa..."
Any show that brings some kind of representation is going to be VICIOUSLY defended by social justice circles on tumblr, even if it's meant for kids to enjoy.
SU in particular because it hits soooo many marks.
-First CN show made by a woman.
-Majority female main characters.
-Three powerful female protagonists and one male protagonist who isn't treated as inferior or emasculated.
-Several overweight characters and not one fat joke is made.
-Majority POC (people of color) among voice cast.
-The character Stevonnie not having any specific gender.
The gems themselves are a two-fer. They're officially genderless, but are referred to as female, and there are at least two examples of romantic relationships between them.
So naturally the show gets latched onto by a lot of people who do stuff like attacking people who cosplay the characters but don't match the representation that character gives them (skinny Rose Quartz cosplayers, non-black Garnet cosplayers) and act like anyone who doesn't receive representation for it (white people, men) is going to take it away from them.
As sad as it is when someone refuses to watch the show specifically because of how the fandom behaves... I can't blame them in the slightest. Sometimes you just have to avoid places where fandoms go and enjoy the show for your own reasons.