My guess is just accept that everyone is trans and gender is a construct for the story. Eve (if you think about it, name based on the first woman created from a rib of a man) is a position. The only reason it has to be a woman is because of the culture that a saintist must be a woman and a demon king must be a man. That's why they don't get to keep their bodies, because the gender is part of the hederonormative conception of the character. So another way of defying them, is by remaining what you already are, if that makes any sense?

People are commenting that we're supposed to be piecing together that Eve and Kiri switched places, but I'm still not understanding how that works when they are both guys and the original Saintess was a woman. I understand that a guy could overwrite the role, so Kiri could stay a guy as the Saintess, but Eve was supposed to be a woman who now became a guy because of switching roles? I like the concept, I just don't understand the reasoning behind it. I am very curious about what caused this switch, though. Eve said he saw the guy Gozu replaced before Gozu replaced him, so Gozu obviously wasn't the catalyst.