My sister and I both went to an all girls secondary school.
We had no brothers. Our dad is lovely but wasn't really there much when we were growing up, and we didn't have any male cousins that we were especially close to. My point is that we were seriously lacking in meaningful contact and relationships with boys and men.
I think this shapes you as a person.
Even though my DDs have a brother, and a very loving and hands-on father, I would not send them to a single sex school.
My sister did consider sending her boys to a single sex school, (in spite of it being single sex not because it was single sex iyswim,) because academically it was a better school. In the end she decided against it. She reasoned that the pros in terms of academia were outweighed by the negative impact the lack of girls their own age would have on their life skills.
My DD (17) has gone through a mixed high school. It warms my heart that she has a really good mix of male and female friends, and can relate to boys her own age much easier than I ever could. I have no male friends even now, and they were always a bit of a mystery.
I think for girls, going through puberty around boys is much healthier - they get 5 years to learn that there are many fabulous teenage boys out there who will tun into fabulous men, and they get an insight into the fact that some are knobs who you shouldn't trust or touch with a barge pole!
I wouldn't want my DD to be discovering that for herself, miles from home on a university campus at the age of 19. They also learn that not all boys are out for sex, and that you don't need to 'put out' to be seen as cool or to be accepted, and they learn to see the opposite sex as something that is part of everyday life!
There is no way I'd send any of mine to a single-sex school, OP!