Okay, what if we go back to Mars for male, Venus for female. Then a tiny unicorn head for Androgyne, a fancy GQ for genderqueer, and a question mark for questioning. I'm not sure about intersex, but we could poll people or do a combined Venus-and-Mars symbol (hope that's not offensive).
I'm happy to be represented as just male rather than FTM. So back to the Mars symbol would work for me.