Dude, it's SO WRONG they locked your thread on this topic! The show gives us all these sexi girls and innuendo, and now it's illegal to ask the simplest question about who's doin whom! Now all we have is this one thread, where there's already halfadozen topics going on.
Anyway, I want to contribute to your DOING THE DEED BREAKDOWN:
Certainty:
Ozai >>>> Ursa. At least twice.
Katara's Dad >>>> Kya. Also at least twice.
Probability:
Zuko >>> Mai.
Sokka >>> Suki.
Speculation:
Long Feng >> Joo Dees. Not all of them, only the hottest ones.
Iroh >> Jun (come on, she was paralysed!)
Outside chance:
Ozai > Azula (Electra complex)
Azula > Ty Lee < Mai
Earth King > Bosco or vice versa o_O
Set for the future:
Aang > Katara
Toph > Haru (yes in that order)
Wishful thinking:
Zuko >>> Azula (there's practically fire between the two)
Every Hundred Years, and also Inbetween:
Fire Nation >>>> Everyone Else