My DD is now at her second school that was originally all boys but switched to co-ed (she's Y9, 10 in sept). Primary was independent, secondary is state. Both schools changed around the same time as one another.
I would say it depends entirely on your child and on the school and how they handle it.
At primary, the ratio in her cohort was about 3:1 b:g. With three form intake, there were still plenty of girls for children to make friends with, and generally all the girls had one or two very close friends without problems. DS attended the same school, and although his year had the same ratio initially, more girls than boys left over the time from age 3-11, and there were some girls that found it harder to have a close special friend, though all were friendly with all the girls in their year. Other cohorts in between my children had a more balanced mix of g&b, overall the school is about 2:1 boys:girls.
I would say the only time she experienced issues of bias was with the behaviour of the oldest boys- there was for quite some time a lenient approach to curbing the rowdiness of the eldest boys (boys will be boys). However, that was really clamped down on in the year she was top of the school, and certainly in DS's cohort (3 yr gap) there was absolutely none of that. The boys in his year were very polite and well behaved and didn't throw their weight around in any way. Academically, the girls were pushed just as hard as the boys.
There was one issue of sexism with an extra-curricular activity in Y3, run by an external body, and our complaint was taken seriously and the external provider actually changed.
DS often opted for extra-curricular activities that were heavily dominated by girls (e.g. dancing, tapestry (!), yoga) but was always welcomed and strongly encouraged, so we were happy with that.
The secondary also is about 3:1 boys:girls, with variation between year groups again. There have been no issues whatsoever with expectations of performance or behaviour. DD is taking subjects that are traditionally "boys' " subjects, but has been encouraged and supported at every step.
The only surprise for me was that PE is mixed! I was at a co-ed comprehensive, and PE/Games were always separate, so it seems very odd to me to have boys and girls doing gymnastics, dance, athletics, badminton etc all together. Rugby, football, and netball are separate. Hockey is mixed.
As their primary was independent, they always had games separate too.