Within the gay community there's some animosity towards straight people. Being a part of lgbt community for a few years, I've been exposed to a lot of stuff like "fucking straights" in a poisonous, callous manner. It started out as an insult. Also see "baby havers". It's in the line of minority "we're not accepted so let's marginalize and pretend we're better" thing.
Never mind that most people don't even know what CIS means, whereas straight is a word in common use. So when you call someone cis, then they'll be like,"WTF is that?" and then you have to explain it and shit, rather than getting to your main point. It creates an us vs them thing, which is largely unnecessary when you're trying to be seen as fucking human.