Gender is actually a societal construct. Sure, biologists can divide most species into male and female due to the structure of the body, but gender is a cultural thing. Before colonialism, many Native American tribes recognized up to seven genders. There's a tribe in Sumatra, Indonesia that recognizes five. Now, most modern-day post-globalization people can only conceive of those genders as combinations or variants of two.
With Wicca (correct me if I'm wrong) the gender binary essentialism is a cornerstone of the fertility-based religion. (Although Christopher Penczak among others has written about Wicca's empowerment of gay and queer people, unlike some world religions that condemn anything outside the gender binary.)
Going back to the Bissu of Sumatra, to be transgender isn't only "something that has come up" but seems to be a requirement to becoming a spiritual authority. In the Western world, I can only think of the Greek legend of Tiresias who became a spiritual consultant after being "cursed" with a gender swap (and blindness, I think--? Blind seers were an irony that Greek dramatists just loved,) and of Odin learning seithwork (in a culture during a time that, to my knowledge, actively forbade men from learning that craft.)