I went to an excellent selective co-ed, 60% boys. I had friends but the girls could be very, very bitchy and bullying especially in years 7 to 10. Having boys there didnt help as they didnt really want girls hanging out with them until Y10 to Y12.
I'm sending DD to a local comprehensive which is all girls until Y12. Of course I'm aware of the pitfalls: potentially bitchy and cliquey, not spending time with boys as you grow up. But the academic results are always excellent, there is no gender bias in STEM subjects, there is masses of opportunity in extra-curricular groups for girls. And yes, a big factor is much lower chance of sexual harassment when not faced with boys every day.
The alternative is a very well-regarded, Ofsted outstanding co-ed comprehensive, whose intake is 60% boys, where the results are less impressive and not a single girl is sitting Computer Science or Further Maths A' level in Y12 or Y13, and yet all the pupils taking English Lit A level are girls! The school blame this on self-selection and say there's nothing that can be done about it, they have tried but girls just don't want to do maths or computing. I got told by the senior Computer Science teacher that the only girls who ever opt for the subject "have to be a very strong character and prepared to put themselves out there"... why? I can only guess because they are stuck in a classroom of boys who behave in a way that is intimidating and off-putting. In the neighbouring girls' school the STEM subjects are very popular, it doesn't occur to the girls there that whole areas of the curriculum are off limits.
Plus the co-ed school has gone down the rabbit hole regarding Trans children, and is fully supportive of social transition and I'm really not there yet.
It is depressing as I would probably ideally like a co-ed for my DD but I'm not putting her in an environment that is possibly even MORE gender-stereotyped than when I was at school!
I went to uni with lots of boys and girls from single-sex schools, yes a few (notably from the big-name public schools) were awkward or behaved in a peculiar way around the opposite sex, but mostly these were confident, well-adjusted and not really different to the other people I knew. And obviously hadn't lived in complete isolation from the opposite sex, via siblings and extra-curricular activities.
On balance I've come down "for" all girls schools, to my complete surprise. And I'm glad we have the choice in my town.