I’m afraid that if we don’t get single-sex spaces back soon, we never will.  Allowing men into women’s prisons and locker rooms is still a weird, new (“cruel and unusual,” if you will) idea and can be opposed on those grounds.  If we let male presence in spaces where women are undressing and/or vulnerable become the norm, and stay the norm for very long, single-sex spaces will be the “new” idea and our only arguments will be that single-sex spaces benefit women, which no one cares about.  There will never be money on our side, either.