Quote Originally Posted by Tharne View Post
But genders matter in Viera lore, a lot actually.
from ff wiki: "Final Fantasy XIV gives exposition to the elusive male viera, who are virtually identical at birth to females; only upon maturing they are able to be identified gender wise."

So there is lore saying they are almost identical.
Can make up lore saying that some males choose to dress/live as "females" to remain with them. And others choose to be more masculine and leave.