Calling someone "cisgender" is like calling someone a 'muggle'.
A muggle is a just non magical person. Technically we are all muggles because none of us have magical powers.
However, when you call someone a "muggle" you are implying that you think some people really do have magic powers. You're advertising the fact that you're an idiot.
Same with "cisgender". It technically means 'someone who's gender is congruent with their sex.'
Thing is, this is true of everyone! Even those people who believe that they have transitioned gender.
When you use the term "cisgender" you are advertising the fact that you believe some men have become women. You're being a bloody idiot.