Lucydo, many of the girls may feel fine, but they may be concerned for Muslim, orthodox Jewish, or overweight/struggling with body image friends.
Last year the girls on DDs University course organised a girls only night out, so that their Muslim coursemates, several of whom were living at home, could join them and feel able to take their hijabs off. I have heard reports that in some Universities, Muslim students are inclined to self-isolate. Small things, like an hours swim session a week, or a considerate girls' night out, will help prevent this.
And this at University. A loss of single sex spaces in a wider society could have a real adverse impact on community cohesion.