The races are genderlocked because the dev team hit a snag with time and resources and could only do one of each race.
This is the real answer here. None of this is about Lore. It's 100% a time/resources debate. As Yoshi P said, each piece of armor in the game has to be remade 2 times now. If they made male Viera and female Hrothgar, they'd have to do it 2 more times. More animations, more everything.

Now, should they have delayed releasing them until they were able to complete the work on both genders for each? Sure, why not. Would people want to wait though? Probably not. Damned if you do, damned if you don't.