"You can't change your appearance again?"
"I can change my appearance, but not my gender," Lu Bela replied.
"How does that work? Don't artificial humans operate by core? Gender shouldn't have anything to do with it, right?"
"That core is filled with data. Thinking of it as DNA. It stores all our information. Once they have selected a gender, the data changes, and their DNA gets modified, just like a sperm cell when it starts ovulating. You can't modify the gender once the zygote has formed.
If I need to give a better example, I would say it's a matter of software and hardware. Our bodies— shells, are hardware, and our data-slash-core-slash-DNA is software. Once we have selected a gender— the software changes and our bodies are only compatible with selected hardware."