What is the third moral principle that forbids gender change?

Shortly after the beginning, Cain kills Abel. (Genesis 4:8) In light of this, God sees it fit to create a new moral rule for all of humanity. 1. "Do unto others as you would have them do unto you." (Luke 6:31) Many thousands of years later, a Prussian philosopher by the name of Immanuel Kant realized that this is not enough. He realized that people would misinterpret this rule to mean that as long as I am not directly affecting others, I can do as I please (non-aggression principle). So in order to remove irrational, ungeneralizable behavior such as drug usage, Immanuel Kant created an addendum to God's rule. 1. "Do unto others as you would have them do unto you." (Luke 6:31) 2. "Treat yourself as you would have others treat themselves." Now let's suppose that some new alien technology falls from the sky that makes complete and perfect biological sex change possible: you step into the portal, and you emerge a perfect, biologically-complete version of the opposite sex. Most Christian theologians would conclude that you are forbidden from using such technology. So what's the third rule? The gender-changing alien technology doesn't violate Rule #1 (nobody is harmed, unless you consider any kind of self-change to be "harm") and it doesn't violate Rule #2 (society would be a lot more fluid, but ultimately, the human race could continue to procreate if e.g. a gay man chose to turn into a woman, so it's not self-destructive in the way ununiversalizable behavior like drug usage would be). So there must be a third hidden, unspoken rule of morality that Jesus and everybody else missed. What is that rule?