^^ I'm sure you know this, but all trans issues don't boil down to genitalia
I'm assuming that in this hypothetical, the guy in the fraternity is FTM (or vice versa for the sorority example). If that's the case, let him be.
I can't remember if it was discussed in the other thread, but I'm not sure what the (single sex status) ramifications are if someone is MTF (or vice versa) in a fraternity (or sorority). I'll have to check that out. Thanks for the link, amIblue.