Because you're sad for the staff, or sad for the men?
The staff, of course! The FEMALE staff.
I've made it clear above that it puts them in an impossible position, having to enforce the necessarily rigid door policy that a strictly single-sex pond would necessitate. (No one has yet given me a good example of how such a policy could be carried out in practice, without sacrificing any women's safety and/or dignity. Someone suggested a male bouncer, but I bet most women don't want to be eyed up by a man assessing their claim to femaleness as they go in. Plus, trans men who wanted to swim in the Ladies' pond might be given an awful time at the gate, which would be, well, awful.)