I've seen both sides of the single sex school. I went to a girls' grammar school in the 1970s - very traditional and old fashioned, like a convent without the nuns. We were expected to behave like young ladies and I hated every minute of it. I was bullied, taunted, ostracised and generally made to understand how worthless I was, by staff and girls alike.
Interacting with boys was nigh on impossible. I didn't have brothers and was too scared to go to youth club (a. I'd be laughed at, and b. I was told by the adults in my life that youth clubs were veritable hotbeds of teenage debauchery), so by the time I was 16, boys were foreign territory to me.
Gradually, thanks to the bloke I married and also to having a couple of sons, I learned to be comfortable around men and boys, and actually have better male friends than female these days - maybe because I don't feel I have to compete with them.
I vowed at 16 that no child of mine would ever attend a single-sex school, yet ended up sending both of my lads to an all boys school, simply because it was a good school that cared for them and nurtured their talents. They did, however, join out-of-school clubs and activities where socialising with girls was a regular thing, and I like to think they have both developed healthy attitudes towards women.
Lil Ol' Lady has it about right - the right school is more important than whether it's single or mixed sex. I guess my school was the wrong one for me - it was very traditional and I'd been to a somewhat progressive primary. I was never going to fit in there. I would just advise any parent thinking of a single sex education for their child to make sure they get lots of boy/girl social interaction outside of school.