A: Cis people aren’t merely “normal” because this language is stigmatizing and insulting to trans people, implying that we’re abnormal. The normal/abnormal distinction in reference to people has strong connotations: that being normal is desirable, while being abnormal is vicious and reprehensible. By implicitly perpetuating the view that being trans is somehow morally wrong and/or dangerous, this language contributes to misunderstanding, injustice, and violence toward trans people. Socially, it’s often a slippery slope from calling a group of people “abnormal” to calling them things like “freaks,” “perverts,” and “monsters.” (Saying cis people are normal is also insulting to cis people: Who says you have to be boring and ordinary just because you’re not trans?)
The cis/trans (etc.) distinction, on the other hand, is unbiased and scientifically legitimate. The terminal goal in using language that doesn’t privilege anyone is to eliminate cis people’s social and political privilege, and thus to put trans and cis people on equal footing.
So the word “cis” has a clear, neutral, and academic purpose; it fills a conceptual gap in our language. In other words, “cis” in the context of gender is analogous to “heterosexual” in the context of sexual orientation: It means something specific, and it’s not pejorative toward either those who do fall into the domain it picks out or those who don’t.
In our society, everyone is assumed to be cis unless and until they indicate otherwise, and this social fact is built into the concept “cis.” Accordingly, unlike words that refer to specific gender identities (like “man,” “woman,” “agender,” “genderqueer,” etc.), “cis” isn’t a term with which you have to identify in order for it to be accurately applied to you.