Well, not getting the distinction is pretty important. People can't discuss an issue without both understanding the definition of terms, or we'd just be speaking different languages to each other.

Cis and trans are both Latin prefixes.

Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of".

Cis- The prefix “cis-” comes from the Latin meaning “on this side,” as opposed to “trans-” which means “on the other side of” or “beyond.”

So all cisgender means is not transgender. Or transgender means not cisgender. Neither are insults that should offend, and if one does, it might be a viewpoint thing.

You are, presumably, a homo sapiens sapiens. We don't use that term generally, and just call you human. However, both are correct labels, and they aren't insults. If there were homo neanderthalensis around still, the distinction may come up in conversation for various reasons, and that's when it'd be necessary to have the different labels.

Straight or gay/bi/lesbian/pan/ace/demi etc are all talking about something completely different, which is your sexual orientation. Calling you straight when we're discussing whether you are cis or trans would be like calling you lotion or something when talking about your species. Lotion isn't a species, so it's pointless to the conversation, and just plain wrong.

You could of course request to be called lotion, but it's a confusing title if someone was discussing your species, especially medically, in socially (as in social issues or norms for your species), issues that pertain to your species, medication, etc.

Now, here is where I might be skirting close to accusing you of something, but if possible, please just think deeply about it. Why is CIS unacceptable but straight acceptable? Is it possible that CIS makes you not feel "normal" as a label, while straight does?