1. The basis of all of this, the stark rise in reported sexual offences, was due to an aggressive campaign to increase reporting of such crimes back in 2013. This comes directly from the agency that collects these statistics.
2. The proposal, intentionally or otherwise, panders to fundamental religious types. This particular candidate is known for his extremely strong religious views - very unusual in the UK for anyone, let alone a political leader, since it is one of the worlds least professed religious countries.
3. It creates a segregated society that simultaneously victimizes both sides and changes their perception everywhere else. If segregation is appropriate there, suddenly everyone will frame problems in such a light and therefore why not everywhere else? If it is appropriate between genders, then why not other profiles?
In short, it is an extremely dangerous, impractical yet insignificant idea (in terms of overall effectiveness) that has zero place in any open society such as most Western societies.