Perry, first of all -- if you are gender fluid you're not cis.
Second, when it comes to "cisphobia" (if that's what want to call it). For me this takes the form of a certain reluctance or tiredness to deal with people I'd describe as "cissexist". People who question my experience, people who don't "get it", who take a biologist stance towards gender, etc.
But even if you ended up identifying as cis, you would still have a much better understanding of how it feels to question your gender (and have it questioned by others). And you most certainly wouldn't fall into the cissexism trap because of your experience! So as you said -- you would still be able to offer valuable support to folks in a similar situation. Why wouldn't you not be welcome in the community
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