Is cis or trans identity really something that is truth-apt (& therefore in the purview of probability)?
Could you explain what you mean by this via an easier to grasp concept than gender identity, preferably in a way that preserves relevance to identity?
Sure. Does it make sense for an individual to think about the probability that they (themselves) are a Manchester United fan?
I say it doesn't, really. If you (a) like ManU in some sense, and (b) are willing to call yourself a ManU fan, you are a ManU fan.
Rationality quotes time!
The usual rules: