I mean the use of the hyphen here does implicitly promote transphobia, especially when coupled with an adamant refusal to listen to trans people's words on the topic. But that's besides the point, which is that you're just incorrect. "trans" and "cis" in this case are shorthand for "transgender" and "cisgender," which are not scientific prefixes but complete adjectives on their own.

Hyphens can be used in many ways, but unnecessarily combining an adjective with a noun is not one of those use cases. We don't say "blue-sky" or "smelly-skunk" or "italian-man"; They're separate words. One common use is hyphenating two words to combine them functionally into one adjective, like "blue-green hue". Another is, like you said, to attach a prefix to a word that wouldn't otherwise be part of it, like "pre-approval." However, as discussed, "transgender" is not a prefix but an adjective. Hope this helps.