You believe that cis is a slur because that's how you use the word trans.
These people are so fragile. If you dare remind them that they're cisgender, that they're straight, that they're white, that they're able-bodied, they take it personally because to them, they're just normal.
All of the sudden, they have to think about their place in society and it absolutely terrifies them. They are like rich people when you tell them that they're rich. They will do this whole fucking song and dance to try and tell you about how they are not that rich and how the government taxes them and everything and how they're just like you and so on.
They are not white, they are not cis, they are not straight, they are not able-bodied, no, they are just normal. It's everyone else that is different. It's them who are black, who are gay, who are trans, who are disabled. They're different.
But my guy, you are not. You are as much a part of the society as I am. And you're getting just a small taste of what it's like to be a minority in said society. Someone reminded you of what you are for just a second. Now, imagine being constantly brought back to what you are and not who you are. What that would be like.
People like Elon Musk take offense to being called straight, white, cis or able-bodied because everything else is inferior to them.
On a side note: Someone should remind him of what the Nazis did to disabled people. Because the dude is clearly getting cozy with a lot of them, but I don't think being this level of clinically stupid is going to get him far in the Fourth Reich. Besides, those South African origins... erh, that may get him in trouble. And having a trans daughter? Woof, doesn't sound like "good genes" to me. Elon, you may want to pick better friends.