Quick primer to explain to you. Note - the following is taken from Stonewall's pages, and I believe quoting banned words in the context of a political discussion about their meanings and usage is okay under talk guidelines; it's applying them to a specific poster (usually in contravention of that poster's expressed preference) that is not okay.
So here we go:
Gender
Often expressed in terms of masculinity and femininity, gender is largely culturally determined and is assumed from the sex assigned at birth.
Gender expression
How a person chooses to outwardly express their gender, within the context of societal expectations of gender. A person who does not confirm to societal expectations of gender may not, however, identify as trans.
Cisgender or Cis
Someone whose gender identity is the same as the sex they were assigned at birth. Non-trans is also used by some people.
Join the dots on this set of definitions and what you in fact have is an acceptance that gender is the set of culturally determined sex stereotypes (the ones feminists believe are the tool used to oppress women and keep us in little restrictive boxes) and that being cis involves being comfortable with the set of restrictive sex stereotypes used to oppress you. To tell a woman she is "cis" is to tell her that she is choosing to collude in her own oppression.
That's why we don't like the term very much, and that's why MN has said it's not to be used to describe particular posters. (Incidentally I don't agree with that ban - I find the term offensive, but I'm perfectly capable of articulating why it's so offensive, and if people then choose to continue to use it, they're the ones who then look like the unpleasant gits they are, not me. I think as a general rule censorship and language-policing are a bad thing.)