I went to a single sex secondary, and if I had a daughter, I would order it. There was some bitchiness, though not tons, and I suspect that also happens in mixed schools (and now I work in a male dominated industry, I suspect it happens in boys schools, too - it just gets a different name.) It could also be very supportive.
I'm still friends with people I was at school with and this is the year we turn 50. Quite a few of us work in STEM roles.
It's not like we never saw boys. We saw them on the school bus, in town, at youth clubs, swimming club, DofE, and so on. 6th form had shared classes with the boys school for some subjects, and there were some shared drama productions and language exchanges and stuff.
Last week, I went to the pub for a colleague's retirement - I was the only woman in a group of about 15. I then spent a weekend away with a similar number - all women. Both experiences were good in different ways. Whether any of this was a reflection of me having had a single sex education, I can't say, but it didn't kill my ability to mix with men. Or women.
All schools have different cultures, and all children are different - a good school, whether single sex or mixed, won't suit every child. It's great to have a choice for that reason - but some areas, especially smaller towns with rural catchment areas don't havea large enough population for that to be an option.