To be fair to benders, the fire nation pretty much almost wiped out all the other benders during Aang's time. And *DID* wipe out all of the air benders, except for Aang. So I think that at least one parent was a bender. I mean, isn't the idea of the extermination that once the benders are eliminated, there won't be a way to get any more? At least, that's how it works out in my mind. I guess we'll see!