First, being awfully authoritative over something which is not only fictional, but a highly improbable situation.
One of the rules about decent fiction is to not only establish rules for the world being built, but to occasionally *break* at least one of those rules.
The Rule I'm talking about is not so much a spirit-induced pregnancy between two women, BUT a possible ONE-OFF exception to where Raava goes after Korra.
Hell, IF Korra were the birth-mother, there is the possibility for Raava to be forcibly transferred upon the birth of the child, leaving Korra no longer the Avatar. Before you say that could never happen, remember that Raava was forcibly removed from Korra once. By a spirit. Now, sure Vaatu is Raava's mirror counterpart, but who is to say there are no other spirits able to do the same.
WILL it happen? ::shrug:: who knows except whoever might write such a fic. Keep in mind none of us thought Raava could be forcibly separated from an Avatar—until it happened. Until then, we thought death was the only separator.
It certainly will not happen if no one is creative enough to see and seize the possibility.