Stephen, I agree with the “official” ruling 100%. A case by case assessment sounds perfect.
Now that it’s been settled that I’m not a dirty dirty cheater.

I really want to give some advice to players who are thinking about making oppositely-gendered characters. (Please note I’m saying oppositely-gendered and not oppositely-sexed.) As someone who identifies as female and played a male character for 6 years, I would NOT recommend doing this unless you are really, really serious about it and prepared for the consequences.
Who am I to give advice? I think I am the most commonly referred to PC in this thread. I play Kovu. Kovu was a male Koala. I have played Kovu in 6 different chapters. From my experience this is what I can tell you about playing the opposite race:
1. Boys didn't flirt with me. (Hoorah! That was my goal.)
2. Every chapter is different about how the players react. I mostly saw confusion.
3. I got RP about "how to be a better man." I got really upset at some people for thinking I wasn't a good enough man. This nonsense would have never happened if I wasn't OOG female. (or maybe I just don't understand boys?)
4. No one ever asks “What do I see?” with a PC. People assume that if they see a feminine face, the character must be female. Be prepared for this.
5. Find some good friends to back you up. My teammates were always amazing; they would correct others pronoun mistakes so I didn't have to spend all day saying, "I'm a boy."
6. Even if they knew better, people messed up my pronoun forever. (heck: sometimes I messed it up too!)
I think it is very possible to play an oppositely-gendered character. It’s a fascinating “be all you can’t be” moment. I had lots of FANTASTIC roleplay because of it. But honestly, I’m glad it’s over. (My race change ritual backlashed for customer service reasons. I can't phys rep being a boy while playing a MWE. No one would believe it. And I was getting really tired of playing a boy.) It will be nice to walk naturally, use my upper vocal range, wiggle my figures when I talk, and, most importantly, giggle without getting funny looks.
I would love to talk to anyone who is considering an oppositely-gendered character. Please feel free to PM me with questions. Cheers.
-Sam