Well, “tumblr” doesn’t hate cis people. Most people on tumblr are cis. And many people on tumblr are transphobic. And many are anti-sj. Tumblr doesn’t have one hive-mind. I’ve met neo-nazis, white supremacists, conservatives, trans misogynist radical feminists and virulently transphobic people here and certainly none of them hate cis people—in fact, most of them are cis.
You probably mean “why do trans people on tumblr hate cis people/use the word “cis” so negatively?” For one thing, the word “cis” was not invented on tumblr and tumblr is not the first, nor the only place the term is used. It’s the first place, however, that many cis people have come in contact with this term and (knowingly) come in contact with trans people and trans discourse. For another thing, most trans people on tumblr, and most trans people in general, do not “hate cis people.” But more on that later. First:
1) You weren’t born in a “female body” and saying so is cissexist. We are assigned female or male at birth and our bodies tend to be coded as female or male in this society.
2) “Cissexual” is not a real thing. The term is “cisgender” (“cis” for short). And you’re right: being cisgender means that your gender matches what you were assigned.
There is no malicious meaning behind “cis.” The only reason you’re encountering hostility toward cis people and angry statements about cis people from trans people is because we are venting about our frustrations with dealing with oppression and ignorance at the hands of cis people. Also, trans people often critique cis people as a privileged social class. You as a cis person have cis privilege and you probably have said and done things that have hurt trans people. Saying you have a female body hurts trans people, even if only in a subtle way. Even complaining about tumblr “hating” cis people hurts us because it recenters the discussion on you instead of on all the laws and social institutions and real world discrimination and erasure that trans people face. It makes the discussion all about your feelings.
But I get that if you’re new to trans issues and don’t get the context of the angry posts trans people make, so you might not understand the connotation of the word “cis.” But you’re also missing that we use the word “cis” in a lot of neutral ways too, just to talk about you as a class in society or refer to you or talk about a trans person and cis person interacting, etc etc. I guess you all don’t pay attention to those posts, though. You don’t listen until we say something that bothers you. Then you pay attention. The posts that get all the notes and attention from cis people are the angry posts about cis people. All the posts we make discussion our cis friends or certain issues or just using the term neutrally when we talk about our everyday experiences and observations don’t see the light of day in your circles.
You also don’t seem to understand that without the word cis, so much damage is done to trans people. Otherwise terms like “biological women/men,” “natal women/men,” “women born women,” “real women/men,” etc are used and these terms hurt trans people because they are based on shoddy science that privileges your identities over trans people’s. Cis people who oppose the word “cis” are transphobic.
Cis is a neutral term. it’s like the term heterosexual: it denotes a group, nothing more. That group happens to be privileged and is often seen as the default. A lot of straight people and cis people take for granted their sexuality and/or gender. Trans people’s gender is magnified, but most cis people don’t even see themselves as cis. They see themselves as “normal” or “natural” and trans people as “abnormal” or “unnatural.” I suspect some of cis people’s negative reaction to the word cis turns on the fact many of you would rather continue to think of yourselves and us in these terms, which reaffirms the legitimacy of your identities over ours. Even if that’s all subconscious. I also think you don’t really like the fact we came up with a term for you. Historically, the privileged class has named themselves and controlled language more. Every time I see an oppressed class come up with language to describe themselves and others, I see push-back.
I hope that helps you understand. I suggest you pay attention to what trans people are saying and google some stuff to read trans people’s words because it’ll give you more context to know where we’re coming from.