No matter what excuse they come up with to gender-lock Viera, it's gonna sound forced from story standpoint (or at least the direction it's moving).
How exactly are they gonna make the WoL and their friends heading to the woods where Viera resides but never encounter any male Viera NPC?

And if they actually made male viera NPC in game but not playable by the players (while the female is) then it's even more ridiculous IMO.