GAW – That was different. A) It wasn’t because of the magical field but because she was mistakenly identified as the 7th son of a 7th son and the handing over of the staff is irreversible. B) It was a quasi-sociological issue, not an anatomical or biological one. Same reason there is the occasional male witch…
FYI, it was the eighth son of the eighth son. Even so, “Unless it is the male of Chelys galactica that gives birth” is a large Caveat (as is the species).