So, we can't have transgender headcanons because - why?
The short answer is that the rule is in place to avoid potential conflicts (between muns) involving transgender headcanons.
The long answer is that such types of headcanons are controversial and are common sources of drama that are possible to avoid. Explicitly looking at the characters those headcanons often attach to - Naoto Shirogane, Chihiro Fujisaki and Pidge Gunderson are the three most popular examples - shows far too much outrage within communities, both RP and non-RP, for me to be comfortable allowing them within the group. Another facet of this answer involves an incident from another RP group that had occurred a while back that I’d rather not repeat here (that is to say, I want to neither explain what happened in this ask nor have it happen again in Java).
I apologize if that makes you upset or uncomfortable, but I won’t be changing that rule any time in the foreseeable future.