I still don’t really understand the term.
From context, I think it’s supposed to mean when people who aren’t trans treat trans people badly.
But shouldn’t that be trans-sexism, or sexism against trans? But it’s not really sexism because it’s not discrimination based on sex. It’s discrimination based on gender identity. So shouldn’t it be gender-identityism.
Is the phrase meant to imply that only ‘cis’ people can be sexist? I don’t believe that to be true if it is meant to imply that. Some of the most sexist and stereotypical things I’ve seen or heard have come from the trans community.
Also, it’s not really clear who’s being ‘sexist’ and who’s suffering from the ‘sexism’. It’s kind of like saying white-racism to mean that some white people treat ethnic minorities badly. Because cissexism sounds like sexism against ‘cis’ people to me.
Sexism already exists. If you wanted a word to describe discrimination towards trans people then I don’t believe ‘cissexist’ was the right word to invent. It makes my brain itch because I know it doesn’t sound right. So instead of listening to the argument all I can think is ‘Why did you choose that word?’.