As with all such questions, it depends on the individual child and it depends on the actual schools available.
I was a 'sciency' girl, I went to a mixed GS where I was the only girl or just one other in one O-level and all A-levels. It didn't bother me one bit.
My main socialising with boys wasn't in school, it was brother's friends and out of school.
DD goes to a girls' GS and it suits her down to the ground. In yr6 she was totally fed up with boys not buckling down to work and decided she'd prefer single sex. I think she'd have been like me and not been at all influenced by peer pressure in her choice of subjects, so would have been ok at a coed - but she loves her school, and they do seem not to be under pressure to 'grow up'
too soon.
I worry about girl-only school promoting & supporting especially "girl" interests too strongly, just because that's where the girls interests tend to lie (wider societal pressures if nought else). Like textiles, gym, lacrosse, field hockey, art, drama, dance
Well, as I said, depends on the school. DDs school has a science specialism. They do all those 'girl' things (except textiles) but they also do electronics for 2 years and many do it for GCSE, all do triple science, can do computer science instead of ICT for GCSE, and there are various science/tech clubs and they have a Science Week (last year had Dame Jocelyn Bell-Burnett speaking).