Its always sold as some thing which is hard to enforce yet the current legislation give the offender a clear choice and ownership of the choice.
Police report and a charge of sex offending with a 100% private prosecutions policy by the Board.
Clear notices of the facts.
Eg. That any male in the female pond area (or female in the male pond area) will cause alarm and distress etc ... and if found will have the police called and will be charged with sex offending.
Add a map to the mixed sex pond so the user has access to a lawful alternative.
Then the police sort out ID and the Board action if there is an arrest.
Simple solution.
No big fuss needed.
Non-sex Offenders will opt to use the risk free mixed sex areas.
Sex Offenders get to risk a nice criminal record.