I am a simple person. In years gone by, when I saw our local swimming pool advertise women-only swim sessions, I assumed the only swimmers admitted to the pool during those times would be female. I myself never got round to taking advantage of these sessions, but it doesn't take a huge leap of imagination to see why some women would feel more comfortable exercising and getting changed before/after these sessions without any men present. Many women are intensely self-conscious about their appearance, often because of catcalling and worse from men. Some, sadly, have good reason to fear being near male bodies because of a history of experiencing male violence and sexual violence. Then there are those from strict religious groups who can't exercise publicly if there are men there.
Nowadays if I saw such a session on offer, I would wonder if I could be just as sure as I used to be that it would be female only. And if I wonder this, with no particular reason to prefer single-sex sessions, how much more doubt will there be in the mind of a woman from one of the groups I mentioned above. I would expect many of them to decide not to bother even trying the sessions out.
It's akin to the urinary leash of yesteryear. If women can't be sure single-sex facilities and services are available, many will simply stay at home and make do without.
Zoe Williams does herself no favours by showing her lack of imagination and empathy in this way.