Their point isn't entirely invalid.

I feel like everyone has a right to self identify and label themselves how they wish, or choose to not be labeled at all. It's fundamentally the same concept the trans/etc community has been pushing for for a very long time and it's difficult to justify rejecting it just because we may not like the person making use of it at the moment.

I've certainly seen people attempt (and fail) to use cis as a slur online in an intentional way.

… you think cisgender or transgender is a slur?

Bro, I wrote 3 sentence that say no such thing and yet, you think that's what I wrote. Read it again. Good grief.

A person can say "I don't want to be called that", regardless of who they are. If you don't respect it, you're not being nice.

Is that really that difficult to understand? If it is, maybe you need to go back to 6th grade and start over because your reading level wouldn't even be that.