Stop sex field change.

[RateU]

You can try this plugin (attached).
It takes fid3 as Sex profile field.
Make it as a required-editable profile field.
In registration, it will try to force a user to choose Male or Female only.
In UserCP, whatever the user's fid3 value set, it will try to prevent it to be changed (you can use ModCP or AdminCP to change the value).