I kept the rule verbatim from femslash12, because, when I took over, the mods asked me to maintain the inclusion of trans and genderqueer characters. However, I think they were arguing for more inclusion over less.

The trouble with headcanon, with no offence intended to your own, is that it can get very odd, very quickly. Claiming that Alistair-Athena as gender queer is one thing, but claiming that, say, MCU Loki is genderqueer is another (though of course people are free to interpret Loki as such, you can see where that would get out of hand if allowed here). I suspect why the assigned female rule came from in the first place.

However, your point about excluding genderqueer characters based on binary assigned gender is well taken. Let me talk about it with the other mod, and get back to you.